In June, the Centers for Disease Control and Prevention reported that the Omicron subvariants BA.4 and BA.5 had become dominant in the U.S. The agency revised that statement on July 5 to say that BA.5 made up over half of new cases in the country.

Experts said that, in general, these subvariants do not have markedly divergent symptoms from earlier versions of Omicron. People infected with BA.4 and BA.5 may develop a cough, runny nose, sore throat, fatigue, headaches and muscle pains. However, they are less likely to lose their senses of taste and smell, or to experience shortness of breath, as compared with those infected with Delta or other variants of the coronavirus, said Dr. Peter Chin-Hong, an infectious disease specialist at the University of California, San Francisco.

Dr. Joseph Khabbaza, a pulmonary and critical care physician at Cleveland Clinic, said people tend to experience upper respiratory symptoms “from the vocal cords to the tip of the nose.” Anecdotally, he said, he has seen more patients with painful sinus congestion and severe sore throats who have tested positive for Covid-19 while BA.4 and BA.5 have been circulating. Some of them thought they had strep throat because they were in so much pain, he said.

There also is not yet evidence to indicate that these subvariants cause more severe disease than past versions of Omicron. But BA.4 and BA.5 are more contagious, which means that as more people become infected, both adult and pediatric hospitalizations are increasing, said Dr. Adam Ratner, director of the division of pediatric infectious diseases at Hassenfeld Children’s Hospital at N.Y.U. Langone.

Credit…Ariana Drehsler for The New York Times

A virus that shows no signs of disappearing, variants that are adept at dodging the body’s defenses, and waves of infections two, maybe three times a year — this may be the future of Covid-19, some scientists now fear.

The central problem is that the coronavirus has become more adept at reinfecting people. Already, those infected with the first Omicron variant are reporting second infections with the newer versions of the variant — BA.2 or BA2.12.1 in the United States, or BA.4 and BA.5 in South Africa.

Those people may go on to have third or fourth infections, even within this year, researchers said in interviews. And some small fraction may have symptoms that persist for months or years, a condition known as long Covid.

“It seems likely to me that that’s going to sort of be a long-term pattern,” said Juliet Pulliam, an epidemiologist at Stellenbosch University in South Africa.

“The virus is going to keep evolving,” she added. “And there are probably going to be a lot of people getting many, many reinfections throughout their lives.”

It’s difficult to quantify how frequently people are reinfected, in part because many infections are now going unreported. Dr. Pulliam and her colleagues have collected enough data in South Africa to say that the rate is higher with Omicron than seen with previous variants.

This is not how it was supposed to be. Earlier in the pandemic, experts thought that immunity from vaccination or previous infection would forestall most reinfections.

The Omicron variant dashed those hopes. Unlike previous variants, Omicron and its many descendants seem to have evolved to partially dodge immunity. That leaves everyone — even those who have been vaccinated multiple times — vulnerable to multiple infections.

“If we manage it the way that we manage it now, then most people will get infected with it at least a couple of times a year,” said Kristian Andersen, a virologist at the Scripps Research Institute in San Diego. “I would be very surprised if that’s not how it’s going to play out.”

The new variants have not altered the fundamental usefulness of the Covid vaccines. Most people who have received three or even just two doses will not become sick enough to need medical care if they test positive for the coronavirus. And a booster dose, like a previous bout with the virus, does seem to decrease the chance of reinfection — but not by much.

At the pandemic’s outset, many experts based their expectations of the coronavirus on influenza, the viral foe most familiar to them. They predicted that, as with the flu, there might be one big outbreak each year, most likely in the fall. The way to minimize its spread would be to vaccinate people before its arrival.

Instead, the coronavirus is behaving more like four of its closely related cousins, which circulate and cause colds year round. While studying common-cold coronaviruses, “we saw people with multiple infections within the space of a year,” said Jeffrey Shaman, an epidemiologist at Columbia University in New York.

Credit…Veasey Conway for The New York Times

If reinfection turns out to be the norm, the coronavirus is “not going to simply be this wintertime once-a-year thing,” he said, “and it’s not going to be a mild nuisance in terms of the amount of morbidity and mortality it causes.”

Reinfections with earlier variants, including Delta, did occur but were relatively infrequent. But in September, the pace of reinfections in South Africa seemed to pick up and was markedly high by November, when the Omicron variant was identified, Dr. Pulliam said.

Reinfections in South Africa, as in the United States, may seem even more noticeable because so many have been immunized or infected at least once by now.

“The perception magnifies what’s actually going on biologically,” Dr. Pulliam said. “It’s just that there are more people who are eligible for reinfection.”

The Omicron variant was different enough from Delta, and Delta from earlier versions of the virus, that some reinfections were to be expected. But now, Omicron seems to be evolving new forms that penetrate immune defenses with relatively few changes to its genetic code.

“This is actually for me a bit of a surprise,” said Alex Sigal, a virologist at the Africa Health Research Institute. “I thought we’ll need a kind of brand-new variant to escape from this one. But in fact, it seems like you don’t.”

An infection with Omicron produces a weaker immune response, which seems to wane quickly, compared with infections with previous variants. Although the newer versions of the variant are closely related, they vary enough from an immune perspective that infection with one doesn’t leave much protection against the others — and certainly not after three or four months.

Still, the good news is that most people who are reinfected with new versions of Omicron will not become seriously ill. At least at the moment, the virus has not hit upon a way to fully sidestep the immune system.

“That’s probably as good as it gets for now,” Dr. Sigal said. “The big danger might come when the variant will be completely different.”

Each infection may bring with it the possibility of long Covid, the constellation of symptoms that can persist for months or years. It’s too early to know how often an Omicron infection leads to long Covid, especially in vaccinated people.

To keep up with the evolving virus, other experts said, the Covid vaccines should be updated more quickly, even more quickly than flu vaccines are each year. Even an imperfect match to a new form of the coronavirus will still broaden immunity and offer some protection, they said.

“Every single time we think we’re through this, every single time we think we have the upper hand, the virus pulls a trick on us,” Dr. Andersen said. “The way to get it under control is not, ‘Let’s all get infected a few times a year and then hope for the best.’”

These subvariants of Omicron are more adept than previous versions of the virus at evading immune protection, whether you have antibodies from vaccination or a prior Covid infection.

“They’re the Houdini of Covid,” said Dr. Peter Chin-Hong, an infectious disease specialist at the University of California, San Francisco. “They’re the escape artists.”

Dr. Stuart Campbell Ray, a professor of medicine at Johns Hopkins Medicine, said some patients are getting reinfected within just two to three months after recovering from Covid, but the symptoms are generally mild.

“Thankfully, the vaccines are still really helping protect people from hospitalization and severe illness,” said Dr. Joseph Khabbaza, a pulmonary and critical care physician at Cleveland Clinic. But when it comes to infection, he added, “it’s hard to view the immunity from Omicron as lasting very long at all.”

Many of the tools and behaviors that help protect against infection can still help you avoid reinfection, Dr. Abu-Raddad said. “There is no magical solution against Covid reinfection.”

Getting vaccinated and boosted, for example, is a good idea even after you’ve had Covid. You only need to wait a few weeks after an infection to get a shot. The vaccines will bolster your antibody levels, and research shows that they are effective in preventing severe outcomes if you get sick again. “Scientific confidence in vaccine-induced immunity was and is much higher than infection-induced immunity,” Dr. Crotty said.

Additional measures, like masking indoors and in crowded spaces, social distancing and improving ventilation where possible, can provide another layer of protection. But because most people and communities have largely dropped these protections, it is up to individuals to decide when to adopt extra precautions based on their risk of getting Covid and how much they’d like to avoid it.

“If you had an infection just last week, you probably don’t have to mask up,” Dr. Adalja said. “But as a month or so passes from your infection and new variants start circulating in the U.S., it may make sense for high risk individuals to do that. People who are trying to avoid getting Covid because they’re going on a cruise soon or because they need a negative P.C.R. test for some other reason may consider taking precautions. Covid protections don’t have to be one-size-fits-all.”

While it’s true that masks work best when everyone around you is wearing one, there is also plenty of evidence showing that masks protect the wearer, even when others around them are mask-free.

The amount of protection depends on the quality of the mask and how well it fits. Health experts recommend using an N95, KN95 or KF94 to protect yourself.

Other variables, such as how much time you are exposed to an infected person and how well a space is ventilated also will affect your risk.

“I know everyone talks about planes, but I would say buses are probably the riskiest, then trains and then planes, in order of highest to lowest,” said Linsey Marr of Virginia Tech, an expert in airborne transmission of viruses.

“People talk about planes having great ventilation filtration, and they do and that greatly helps reduce the risk of transmission,” she added. “But the virus is going to be in the air, no question in my mind, because there’s still large numbers of cases.”

There is also real-world data supporting the protective effects of masking in other indoor locations. A 2020 investigation of a hotel outbreak in Switzerland, for instance, found that several employees and a guest who tested positive for the coronavirus were wearing only face shields (with no masks); those who wore masks were not infected.

And a Tennessee study found that communities with mask mandates had lower hospitalization rates than areas where masks weren’t required.

A number of laboratory studies, too, have documented that a mask protects the person who is wearing it, though the level of that protection can vary depending on the type of mask, the material it’s made from, the experimental setup and how particle exposure was measured.

But the bottom line of all the studies is that a mask reduces the potential exposure of the person wearing it.

Credit…Getty Images

Dr. Marr recommends wearing the highest-quality mask possible when you can’t keep your distance or be outdoors — or when nobody around you is masking up. That means an N95 or equivalent.

Sharon Otterman contributed reporting.

The fast-spread of Omicron and its subvariants has prompted many people to try to upgrade to a higher-quality mask. But that’s easier said than done.

Anyone who has shopped for a mask online or in stores has discovered a dizzying array in a variety of shapes, sizes and colors. Knowing which mask to pick and making sure it’s not a counterfeit requires the sleuthing skills of a forensic investigator. And once you choose one, it’s still a gamble; many people discover they’ve ordered a mask that’s too big or too small for their face or just doesn’t fit right.

“No one has made this easy, that’s for sure,” said Bill Taubner, president of Bona Fide Masks, the exclusive distributor in the United States for both Powecom and Harley KN95 masks, which are from China. “A lot of people end up doing a lot of research.”

Unlike cloth masks, high-quality masks — called N95s, KN95s and KF94s — are made with layers of high-tech filtering material that trap at least 94 to 95 percent of the most risky particles. Early in the pandemic, high-quality medical and respirator-style masks were in short supply. Now the problem is there are so many different masks for sale, it’s tough to know which ones have been tested and certified by government agencies, and which are counterfeit. Testing studies have found that many counterfeit masks don’t even offer the same level of protection as a cloth mask. We interviewed mask manufacturers, importers, public health officials and independent researchers for advice on choosing a quality mask. Here’s a guide.

Masks come in different shapes and sizes. You’ll find “cup” style masks, “duck bill” masks and “flat-fold” masks. The best mask is the one that fits snugly against your face and is comfortable. Start by ordering in small quantities and try different styles to find the best one for your face. Many masks are described as “one size fits most.” But some come in small or larger sizes. “You’re not getting the full benefit of a respirator if you put it on and it’s not forming a seal to your face,” said Nicole Vars McCullough, vice president for personal safety at the 3M Company, the largest U.S. manufacturer of N95 masks.

N95 respirator

Credit…Sarah Kobos/Wirecutter

The N95 respirator mask is regulated by the National Institute for Occupational Safety and Health (NIOSH), a division of the Centers for Disease Control and Prevention. Almost all N95 masks use head straps — two elastic bands that wrap behind the head. If a mask claiming to be an N95 has ear loops, it’s most likely a fake. The C.D.C. has a guide for spotting fake N95s.

KN95 respirator

Credit…Sarah Kobos/Wirecutter

The KN95 is similar to the N95, but it has ear loops and is made to meet Chinese standards for medical masks. Some people prefer them for comfort, and because they come in smaller sizes. While you can find legitimate KN95 masks, the supply chain is riddled with counterfeits and there’s little regulation or oversight of the product. One study found that 60 percent of the supply of KN95s in the United States are counterfeit. Keep reading for ways to spot them.

KF94

Credit…Sarah Kobos/Wirecutter

The KF94 is a high-quality mask that folds flat and is made in Korea. It is designed specifically for the consumer market. The KF stands for “Korean filter,” and the 94 means it filters 94 percent of particles. The masks are heavily regulated in Korea, which lowers the risk of counterfeits. However, some fake masks made in China may be labeled KF94, so shoppers still need to do their homework.

Masks for children

The mask market is particularly tricky for parents trying to find masks for children. No N95 mask has been approved for children, so any mask that claims to be an N95 for kids is a fake. However, N95s do come in S/M sizes that might work for some older children. KN95 and KF94 masks have styles made for children, so once you find one, you need to go through the same vetting process that you would use for an adult mask, using the links below.

Big retailers like Home Depot and Lowes typically work directly with manufacturers approved by NIOSH or their distributors, so if you find an N95 mask in a major retail store you can be confident you’re getting the real thing. It’s a good idea to check manufacturer websites to see where they sell their products and who their authorized distributors are, Dr. McCullough said. 3M has a dedicated spot on its website to help consumers spot fake masks.

Finding a reliable mask on Amazon is trickier because you’ll see legitimate masks mixed in with counterfeits, although the differences won’t always be obvious. If you must use Amazon, try to shop directly in the on-site stores of mask makers like 3M or Kimberly-Clark. (You can usually find a link to a maker’s online store right below a product name.)

If you’re buying a KF94 on Amazon, look closely at the packaging to make sure it’s made in Korea and includes the required labeling (see below for more details). Aaron Collins, an engineer who routinely tests masks and who has gained a YouTube following as “Mask Nerd,” recommends buying KF94s from Korean beauty product importers like Be Healthy or KMact. Once you learn the names of a few KF94 manufacturers, you can try to find their websites to learn where they are sold. For instance, Happy Life lists its five U.S. distributors on its home page.

You can sometimes find N95 and KN95 masks for sale directly on the website of a mask maker, like Demetech and Armbrust USA. You can also look for companies that are exclusive distributors of KN95 masks, like Bona Fide Masks. The nonprofit site Project N95 is also a reliable place to shop. Many trusted sites are overwhelmed by demand right now, so you may encounter delays in fulfillment and shipping times.

Legitimate N95s and KN95s are required to have specific text stamped on the front of the mask. Although you may find one in a fun color, masks that are printed with fancy designs or don’t have text stamped on them are probably fake.

Your N95 should be stamped with “NIOSH,” as well as the company name, the model and lot numbers, and something called a “TC approval” number, which can be used to look up the mask on a list of approved ones. The C.D.C. has created an infographic showing you the printing to look for on your N95.

Credit…Charlie Rubin for The New York Times

A legitimate KN95 should also be stamped with text, including the name of the manufacturer, the model and “GB2626-2019,” which is a reference to a quality control standard approved by the Chinese government. (If your mask has a GB number that ends in 2006, which is a previous standard, just check the expiration date.)

The KF94 won’t be stamped with text, but the package should say “Made in Korea” and include the product name, manufacturer and distributor name. Each mask should be packaged individually in a flat, glossy rectangular wrapper with a textured border. The package will also have an expiration date and a lot number printed on it. (Masks that carry an electrostatic charge all have expiration dates.) If your mask comes from a Korean importer, the information on the package will be in Korean, but many companies have begun to create English-language packaging.

A number of resources have sprung up to help people navigate the mask-buying process. Project N95 is a nonprofit known for vetting its mask suppliers. Mr. Collins, the Mask Nerd, has created a number of lists and resources for mask buyers. You can check out his Twitter feed, his YouTube channel and a spreadsheet he has created of nearly 450 different masks and how they performed in his tests.

Mr. Collins may be best known for his list of children’s masks. While there is no N95 mask for kids approved in the United States, mask makers in China and Korea have created KN95s and KF94s for children, including some with child-friendly colors and prints. Mr. Collins created a video “primer for parents” about finding a high-quality mask for kids that has more than 100,000 views.

“I had retired from mask testing,” Mr. Collins said, noting that he doesn’t receive any compensation for his work. “But I came out of retirement to do the kids video. The only place I’ve seen a list of test data is unfortunately me.”

Wirecutter, a product review site owned by The New York Times, has a guide for buying quality masks, one for buying children’s masks and a list of 12 red flags that might signal your mask is a counterfeit.

It’s not easy, but the C.D.C. has a few lists you can use to confirm a mask has been vetted. A note of caution: If you don’t find a particular mask, make sure you’ve looked it up the correct way, said Anne Miller, executive director of Project N95. For instance, a Gerson N95 mask won’t be found under the letter “G.” It’s listed under “L” because the full name of the company that makes it is Louis M. Gerson.

For N95 masks, go to the C.D.C.’s alphabetical list of NIOSH-approved respirators. You can also look up the TC approval number using the certified equipment list.

For KN95 masks, you can use two checklists from the Food and Drug Administration. The F.D.A. created these lists early in the pandemic, when the agency issued an emergency use authorization that allowed health workers to use KN95 masks because of a shortage of N95s. Now that the N95 supply is adequate, the agency has revoked the authorization for these workers, but other people can still use KN95s. While the list is now a bit outdated, finding your mask on it adds reassurance that it’s less likely to be counterfeit — with the caveat that there’s no longer official U.S. oversight for any of these firms. You’ll need to scroll down to find the list and search box.

Use this F.D.A. list to find KN95 masks made in China, and this list for KN95 masks made in other countries.

Jin Yu Young contributed to this report.

If you are an adult who’s been boosted or are a fully vaccinated child, you don’t need to quarantine after a potential exposure. But you should get tested at least five days after your exposure and wear masks around other people for 10 days.

If you’re unvaccinated, quarantine for five days after exposure and then get tested. Wear masks around other people for 10 days. This also applies to those who are eligible for booster shots but have not yet received them.

— Dani Blum and Nicole Stock

Credit…Ibrahim Rayintakath

What does a negative result on a home Covid-19 test really mean?

That’s the question that has confounded many people who have reached for a home test because they have a sore throat, cough or runny nose. After swabbing their nose and waiting an anxious 15 minutes, the result is negative.

While there’s relief in getting a negative result, there’s also uncertainty. Am I really free of Covid? Or did the test just not detect it? Should I test again? Can I spend time with other people?

The confusion is justified, say testing and public health experts. It stems from a lack of understanding about how the tests work. Rapid home antigen tests look for pieces of viral proteins from a swab of your nose, and they are designed to identify whether you have an infectious level of the virus. But a negative test is not a guarantee you don’t have Covid.

It could be that your symptoms are an immune response signaling the arrival of Covid or another invader. The harder your immune system is working to tamp down the virus, particularly an immune system supercharged by vaccine antibodies, the more likely you are to get an early negative result on a rapid test, even if you’re infected.

“It may be that the virus in your body is having a tug of war with your immune system,” said Dr. Michael Mina, chief science officer for eMed, a company that helps rapid test users get treatment from home. “If you test negative and you have symptoms, don’t assume you’re negative. Assume that the virus has not had an opportunity to grow up yet. The symptoms might mean your immune system is just triggering a very early warning.”

Dr. Mina advises people to take a rapid test on the first day of symptoms. A positive result means you almost certainly have Covid. If the result is negative and your symptoms continue, you should still take precautions, wear a mask and avoid close contact with other people. If you can’t test daily, then wait 48 hours and test again. If you’re still negative but your symptoms persist or are getting worse, you should take another test on Day 4. Or you may want to go to a testing center to take a P.C.R. test, which can sometimes detect Covid a little sooner than a home test, although you may have to wait a day or two for the results.

Experts say that if you have symptoms and continue to get negative results on home tests, it may be that your immune system is doing a good job beating the virus. Or it could be that you have another illness. Either way, you should try to avoid infecting others.

“If you have symptoms and continue to test negative, the chances that you’re infectious with Covid have gone down a lot,” said Dr. Robert Wachter, the chair of the medicine department at the University of California, San Francisco. “But you probably should wear a mask that day because you have something.”

And, remember, the result of your home test is just one piece of information. If you haven’t left the house in weeks, your negative result after a few tests is probably accurate. If you have symptoms and you’ve been spending time in bars or a family member has been exposed to Covid, you should be more cautious, even if the initial results are negative. It may be that you tested too early and that your viral load isn’t high enough to be detected.

When Dr. Jillian Horton, an internal medicine doctor in Winnipeg, started feeling ill, she was pretty sure she had Covid. Her husband had been exposed and had symptoms, too. She decided to conduct an experiment of one, testing herself several times over the course of a few days to track the dynamics of the virus. “With my husband testing positive and myself very symptomatic, I was sure I had Covid,” Dr. Horton said. “I was curious to see what I could pinpoint in terms of when I might flip positive.”

Dr. Horton’s husband became ill on a Friday night, and that evening she tested negative. On Saturday, she began to feel sick and tested herself three times throughout the day. All three results were negative.

By Sunday morning, she woke up and was feeling worse. At 6 a.m. she tested and saw a faint line on the test — what she called a “weak positive.” She took two more tests on Sunday and both were negative.

On Monday morning, she tested again, and the test rapidly turned positive.

What is notable about Dr. Horton’s experiment is that if she had tested at a different time on Sunday, she may never have discovered the weak positive. Her immune system was clearly battling the virus, as evidenced by her two negative test results later in the day.

Dr. Horton noted that testing at the right time to catch a high viral load was similar to putting a net in a stream. If the fish aren’t there, you won’t catch anything. But if you time it so that the fish are plentiful, you’ll catch your dinner.

Dr. Horton said she was concerned that too many people think the tests aren’t working when, in fact, they are a useful tool if you understand how to use them. They are ideal for “ruling in” Covid, but you have to consider more information when evaluating a negative test.

“So often I hear people say, ‘The test is useless,’” Dr. Horton said. “What my experience illustrated is that when you have symptoms, the tests are really ‘rule-in’ tests. I think of those two days when I was so symptomatic. I had one positive test and five negative tests. There was only one moment in there where I was more infectious.”

Linsey Marr, a professor of civil and environmental engineering at Virginia Tech and one of the world’s leading experts on viral transmission, said she assumed her daughter had Covid even after a rapid test came back negative. The child had a fever and sore throat, and she had been exposed to Covid through her gymnastics team.

But testing proved useful for knowing that her daughter wasn’t highly contagious, which helped Dr. Marr’s family know how to manage the risk. “We knew we needed to be careful,” Dr. Marr said. “But we didn’t have to totally put her in jail. The test told us that the viral load was not high enough that we had to lock her in her room and be that worried about all of us getting it.” Instead, the family wore masks and opened windows to improve ventilation.

Kristina Kasparian, who works from home in Montreal, believes she may have gotten Covid from her husband, who is a schoolteacher. They disagreed on whether a home test he took showed a faint positive. But a few days later she woke up with tightness in her chest and a sore throat. Her test was positive, and her husband has continued to test negative.

“It’s great to have this tool, but it’s such a variable snapshot in time,” she said.

Dr. Mina said that despite the limitations, people would benefit from frequent testing any time they suspect they have been exposed, have symptoms or want to be sure they are not infectious before spending time with a person at high risk. He also recommends testing before you start interacting with others again to be sure you’re not still infectious.

“These are tools that have massive benefit during a pandemic like this,” Dr. Mina said. “They will catch you when you’re most infectious. They will even catch you most of the time when you’re just slightly infectious. They will catch almost everyone when they have a high enough viral load to spread. But it won’t be perfect.”

If you’re around other people when you get the bad news, the first thing you should do is put on a mask. Then, isolate yourself as quickly as possible, even if you don’t have symptoms and even if you are vaccinated.

Once you take a deep breath and have a moment, think about where you’ve been and who might have inhaled your germs. Then, let your employer know. Many workplaces have protocols for contact tracing when an employee tests positive. You should also alert anyone with whom you’ve spent time, going back at least two days before you got tested or started having symptoms, said Dr. Paul Sax, an infectious disease expert at Brigham and Women’s Hospital and a professor at Harvard Medical School. The C.D.C. defines a close contact as anyone who was less than six feet from you for 15 minutes or more.

Don’t forget to also tell your doctor, if you have one, particularly if you have an underlying medical condition, like high blood pressure or diabetes, that puts you at higher risk.

If your child tests positive, call the pediatrician. You also need to call your child’s school as well as anyone with whom your child has had close contact with at play dates, parties or other activities.

There are two oral antiviral therapies currently available to treat Covid-19 in the United States.

One, called Paxlovid, was developed by Pfizer and was the first oral treatment authorized for high risk Covid-19 patients ages 12 and older in December. It is prescribed as three pills taken twice a day for five days.

The second drug, called molnupiravir, was developed by Merck and was granted emergency use authorization just one day after the Pfizer treatment in December. Molnupiravir treatment consists of four pills taken twice a day for five days and is available for high-risk adults ages 18 and up.

The Food and Drug Administration has also cleared an intravenous antiviral medication from Gilead Sciences called remdesivir, which is sold under the brand name Veklury, but it is most often administered at a clinic or an infusion center.

To get these treatments you must test positive with a P.C.R. or rapid test. This can be done at home, at a regular health care provider’s office, at a testing site or at one of the pharmacy chains, community health centers, long-term-care facilities or Veterans Affairs clinics participating in the “test to treat” program.

If you test positive at a participating location that dispenses antivirals and has an authorized medical provider, you can get a Covid-19 pill prescription (if you’re eligible) and fill it on the spot. If you test positive at a different testing site or through an at-home testing kit, you can schedule an online or in-person visit with a provider at a “test to treat” location to get and fill a prescription.

And just like with other medications, your regular health care provider can call in a prescription to a pharmacy for you, said Kuldip Patel, the senior associate chief pharmacy officer at Duke University Hospital in North Carolina. You can use the Covid-19 therapeutics locator to find out which pharmacies stock the drugs near you.

Before Omicron, reinfections were rare. A team of scientists, led by Laith Abu-Raddad at Weill Cornell Medicine-Qatar, estimated that a bout with Delta or an earlier coronavirus strain was roughly 90 percent effective in preventing a reinfection in both vaccinated and unvaccinated people. “But Omicron really changed that calculus,” said Dr. Abu-Raddad, an infectious disease epidemiologist.

After Omicron emerged, prior infections only provided about 50 percent protection against reinfection, Dr. Abu-Raddad’s study showed. The coronavirus had acquired so many mutations in its spike protein that newer versions became more transmissible and better able to evade immunity. That means you can catch a version of Omicron after recovering from an older, non-Omicron variant. You can even get sick with one of the newer Omicron subvariants after getting over a different version of it.

Other factors also increase your vulnerability to reinfection, starting with how long it has been since you had Covid. Immune defenses tend to wane after an infection. A study published in October 2021 estimated that reinfection could occur as soon as 3 months after contracting Covid-19. While these findings were based on the genome of the coronavirus and accounted for expected declines in antibodies that could fight off the virus, the study did not account for new variants like Omicron that were radically different from older variants. Because of how different Omicron is, your protection may wane even sooner. In a study published in February that has not yet been peer-reviewed, scientists from Denmark found that some people got reinfected with the BA.2 sublineage of Omicron as soon as 20 days after they got infected with the original Omicron BA.1.

Because the virus is infecting more people now, your chances of being exposed and getting reinfected are also higher, Dr. Abu-Raddad said. And while it’s unclear if some people are simply more susceptible to Covid-19 reinfection, researchers are beginning to find some clues. People who are older or immunocompromised may make very few or very poor quality antibodies, leaving them more vulnerable to reinfection, Dr. Abu-Raddad said. And early research shows that a small group of people have a genetic flaw that cripples a crucial immune molecule called interferon type I, putting them at higher risk of severe Covid symptoms. Further studies could find that such differences play a role in reinfection as well.

For now, you should treat any new symptoms, including a fever, sore throat, runny nose or change in taste or smell, as a potential case of Covid, and get tested to confirm if you are positive again.

The good news is that your body can call on immune cells, like T cells and B cells, to quash a reinfection if the virus sneaks past your initial antibody defenses. T cells and B cells can take a few days to get activated and start working, but they tend to remember how to battle the virus based on previous encounters.

“Your immune system has all kinds of weapons to try and stop the virus even if it gets past the front door,” said Shane Crotty, a virologist at the La Jolla Institute for Immunology in California.

Many of these immune cells build up their protections iteratively, Dr. Crotty said. That means that people who are vaccinated and boosted are especially well equipped to duke it out with the coronavirus. Similarly, people who have been infected before are able to keep the virus from replicating at high levels if they get reinfected. And most people who have logged encounters with both the vaccine and the coronavirus build up a hybrid immunity that may offer the best protection.

The result is that second or third infections are likely to be shorter and less severe.

Dr. Abu-Raddad, who has been tracking reinfections among large groups of people in Qatar, has already started seeing this promising pattern in patient records: Of more than 1,300 reinfections that his team identified from the beginning of the pandemic to May 2021, none led to hospitalization in an I.C.U., and none were fatal.

But just because reinfections are less severe, it doesn’t mean that they are not terrible. You may still run a fever and experience body aches, brain fog and other symptoms. And there’s no way of knowing if your symptoms will linger and become long Covid, Dr. Adalja said.

It is possible that each Covid infection forces you into a game of Russian roulette, though some researchers hypothesize that the risk is highest right after your first infection. One of the risk factors for long Covid is having high levels of virus in your system early in an infection, and you are likely to have such a high viral load the first time you are infected, Dr. Abu-Raddad said. In subsequent infections, your body is better prepared to fight off the coronavirus so you may be able to keep the virus at low levels until it is completely cleared, he said.

Among the many confounding aspects of the coronavirus is the spectrum of possible symptoms, as well as their severity and duration. Some people develop mild illness and recover quickly, with no lasting effects. But studies estimate that 10 to 30 percent of people report persistent or new medical issues months after their initial coronavirus infections — a constellation of symptoms known as long Covid. People who experience mild to moderate illness, as well as those without any underlying medical conditions, can nonetheless experience some debilitating long-term symptoms, including fatigue, shortness of breath, an erratic heart rate, headaches, dizziness, depression and problems with memory and concentration.

Such lingering medical issues are so varied that one study by a patient-led research group evaluated 203 symptoms that may fluctuate or even appear out of the blue after people seem to have recovered.

There is little consensus on the exact definition of long Covid, also known by the medical term PASC, or post-acute sequelae of Covid-19. The World Health Organization says long Covid starts three months after the original bout of illness or positive test result, but the Centers for Disease Control and Prevention sets the timeline at just after one month. For now, doctors must rely on their patients’ descriptions of symptoms and rule out alternative explanations or causes. Some post-Covid clinics have multidisciplinary teams of specialists evaluate patients to figure out the best treatment options.

The picture is still coming into focus, but several studies suggest that getting a Covid vaccine can reduce — but not eliminate — the risk of long-term symptoms. The United Kingdom’s Health Security Agency conducted an analysis of eight studies that had looked at vaccines and long Covid before mid-January. Six found that vaccinated people who became infected with the coronavirus were less likely than unvaccinated patients to develop symptoms of long Covid. The remaining two studies found that vaccination did not appear to conclusively reduce the chances of developing long Covid. In that analysis, one study, which examined about 240,000 U.S. patients but has not been peer-reviewed, found that those who had received even one dose of a Covid vaccine before their infections were seven to 10 times less likely than unvaccinated patients to report symptoms of long Covid 12 to 20 weeks later. But another large study of electronic patient records at the U.S. Veterans Health Administration, also not yet peer reviewed, found that those who were vaccinated had only a 13 percent lower risk than unvaccinated patients of having symptoms six months later.

— Knvul Sheikh and Pam Belluck

Yes. Federal health officials continue to recommend that everyone get vaccinated and boosted, regardless of whether they’ve had Covid-19 in the past.

That’s because vaccines can offer a more reliable and effective immunity boost than a natural infection can.

When you get infected with the coronavirus, your immune system mounts a series of responses that bulk up the body’s defenses against future infections. One of the best ways scientists know how to measure that response is to look at how many antibodies you’ve produced. In general, people who’ve been infected with the coronavirus tend to have lower levels of antibodies than those who’ve been vaccinated, said Aubree Gordon, an epidemiologist at the University of Michigan.

One of the reasons for this difference is that infections trigger many different parts of the immune system, and the size of the antibody response will depend on factors like how much virus you inhaled, whether you have underlying medical conditions and the severity of your symptoms. “You may have a high level if you were sicker or sick for longer,” Dr. Gordon said. “But it’s still going to be lower than what we see with the vaccine.”

Vaccines provide a tailored set of instructions for the immune system to use in the absence of any distractions, such as an active infection, said Paul Thomas, an immunologist at St. Jude Children’s Research Hospital in Memphis. And most people who get vaccinated develop a strong and predictable antibody response. A booster shot reminds the body to bump up its defenses — even faster than the first or second shot — in a matter of days.

Studies also suggest that the antibodies produced after vaccination tend to remain at protective levels for longer.

“I think that’s the biggest argument to get boosted, frankly, even if you’ve had a recent infection,” said Dr. Amy Sherman, an infectious disease physician at Brigham and Women’s Hospital in Boston. “It’s a surefire way to give further protection and make sure your immune system produces peak responses.”

There is no hard and fast rule for when to schedule a booster shot after having Covid-19. The optimal timing will depend on your individual circumstances, including how severe your illness was, how long it’s been since your symptoms resolved and what your risk for re-exposure is.

— Knvul Sheikh

Adults 50 and older and people 12 and older with weakened immune systems are now eligible to receive a second booster shot in the United States. This includes those who have had organ or stem cell transplants, are undergoing chemotherapy for cancer, have advanced or untreated H.I.V. or are on immune-suppressing drugs.

All of the new boosters can be administered four months after your last shot.

One reason older adults may benefit from an additional booster shot is that the immune system tends to weaken as it ages and does not produce the same quantity or quality of antibodies as it did when it was younger. On top of that, older adults often have other medical conditions that take up the body’s attention, putting them at higher risk of severe disease, said Dr. Christian Gaebler, an immunology researcher at Rockefeller University in New York City. “Diabetes, hypertension, obesity and chronic kidney disease are all risk factors for severe Covid,” he said. “And we know that these usually manifest in older age.”

While it takes your immune system about a week to 10 days to mount a strong response to the first series of shots, booster shots should start having an effect in just a few days.

— Knvul Sheikh

The data so far suggest that the side effects in younger kids are milder than those in older kids, probably because a lower dose of the vaccine is given, Dr. Shirley said. Among children under 5, “the side effects were the sorts of things that we might expect in children after receiving a vaccine,” she added, including increased fussiness, sleepiness and pain at the injection site. Comparing the two vaccines, Pfizer’s resulted in fewer side effects overall, probably because it uses such a low dose.

No children in the vaccine trials developed heart issues like myocarditis, a form of heart inflammation that was seen in a small number of older children who received the vaccine, Dr. Shirley said, but this could be because the trials were not big enough to detect rare side-effects. It’s possible there will be some cases once the vaccines are given to enough kids, but experts don’t expect to see a significant number, because myocarditis “occurs more frequently in teenagers and young adults than younger kids,” said Dr. Ibukun Kalu, an infectious disease pediatrician at the Duke University School of Medicine. “I would not expect high rates of vaccine-related myocarditis in the under 5s,” she added. Dr. Kalu also pointed out that the risk of myocarditis is much higher among kids who catch Covid-19 than it is among those who get the vaccine.

It’s important to note that when the F.D.A. did not authorize the two-dose Pfizer vaccine back in February, that was because it didn’t work well enough, not because of any safety issues. (And that’s ultimately why the vaccine now has a three-dose regimen.)

— Melinda Wenner Moyer

The Moderna vaccine is authorized as two doses spaced four weeks apart, said Dr. Debbie-Ann Shirley, a pediatric infectious disease physician at the University of Virginia. Pfizer is authorized as a three-dose series, with the first two shots spaced three weeks apart, and the third shot given at least eight weeks later. (Pfizer’s third shot is not considered a booster. It’s likely that Moderna and Pfizer will eventually authorize an additional booster dose for young children.) Both vaccines are at lower doses than the vaccines given to older kids and adults.

Preliminary data released by Moderna in April suggested that two shots were 51 percent effective at preventing symptomatic Covid-19 infection among children aged 6 months through 1 year, and that two shots were 37 percent effective at preventing infection among kids aged 2 through 5 years. Pfizer claimed that its three-dose series had an efficacy of 80 percent at preventing Covid-19 infection among kids aged 6 months through 4 years old, but that estimate was based on infections in just three children.

— Melinda Wenner Moyer

It’s true that Covid-19 is far less risky for kids than it is for those who are older. But more than 440 children aged 4 and under have died from Covid-19 since January 2020, and the infection is “one of the top 10 causes of death in children in the United States,” said Dr. Yvonne Maldonado, a pediatric infectious disease physician at Stanford Medicine who chairs the American Academy of Pediatrics’ committee on infectious diseases. Also, Covid-related hospitalization rates are higher for children 4 and younger than they are for older children, and more than half of pediatric hospitalizations among kids ages six months through 4 years occur in children with no known underlying risk factors. “I’ve had a number of friends who are health care providers whose children have wound up in the hospital, some on oxygen in the I.C.U., who have no risk factors,” Dr. Maldonado said.

“Vaccines are the most effective way that we have as clinicians to help prevent patients from developing severe forms of Covid,” Dr. Shirley said — and that includes little kids.

— Melinda Wenner Moyer

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