We regularly answer frequently asked questions about life in the era of COVID-19. If you have a question you'd like us to consider for a future post, email us at goatsandsoda@npr.org with the subject line: "Coronavirus Questions." See an archive of our FAQs here.

Readers asked us: How long does it take after getting infected to test positive? When can you unmask after testing negative? And when should I get my booster? And if you do get sick, does that give you months of protection against another infection?

For this edition of the FAQ, we're trying to wrap our heads around the mysteries of COVID time and more.

I tested positive for COVID around Halloween. Now I'm feeling sick and testing positive again. How long is immunity from infection supposed to last?

Getting COVID is no fun, but if there's any silver lining to a case of COVID, it might be that after recovering you'll be immune from getting infected again for some time. But how long until that superpower wears off?

"Typically, we expect protective antibodies to last for several months, though it's always hard to predict when reinfection can happen," says Dr. Seth Cohen, infectious disease physician at University of Washington's UW Medicine.

Exactly how long that immunity lasts depends on a few different things, say our experts.

First, there's the matter of variants. Because COVID is ever-mutating, new variants are emerging all the time – and getting infected by one variant might not protect you against the newest one. "Now that we're dealing with another variant [JN.1], reinfection within a shorter time frame on the heels of a prior variant is certainly possible," Cohen says. Your post-infection immunity from a prior variant may not last as long or be as effective against the newly circulating one.

Data from the CDC shows that the number of JN.1 cases started to rise rapidly in early December. Now it's the dominant strain in the U.S. Someone who got COVID around Halloween was most likely infected by a different variant, so their post-infection immunity could be less effective against the JN.1 strain circulating now.

Another factor influencing immunity is how severe the infection is. Jeremy Kamil, an virologist at Louisiana State University Health Shreveport, says that if you get a mild infection, "your body's not going to respond with its most heavy armaments as it would when you get a [more severe] infection." That latter bodily response is what triggers longer lasting immunity. So if you tested positive but weren't really too sick from your most recent case of COVID, you might not have as much immunity protecting you from reinfection.

Dr. Abraar Karan, an infectious disease physician and researcher at Stanford University, says there would "likely be some protection" afforded from being infected from one variant to another, but adds that post-infection immunity isn't predictable enough to count on for very long.

Let's say I have some symptoms that could be COVID – a cough, a sore throat, fever. Should I test right away to see if it's COVID? Or is it better to wait a day or so? How long does it take to test positive after I first start feeling sick?

For a lot of folks, the first thing they do if they feel sick is to grab an antigen test and swab away. And the result is negative. So ... are you definitely COVID-free?

Well you might have the flu. Or something else. But you in fact might have COVID.

Tim Brown, director of interprofessional education at the University of Georgia College of Pharmacy and a professor of pharmacology and toxicology, puts it this way: "If you get an [antigen] test and it's positive, you have COVID. But if you get a test and it's negative, that doesn't rule out COVID."

The thing is, those at-home tests detect the antigens that build up in your body as the virus multiplies — and it might take up to four or five days for those antigens to register on a test.

Data from a study published last September in the journal Clinical Infectious Diseases suggests that on the first day you feel sick, home antigen tests are only about 30-60% accurate at confirming you're infected. On the fourth day of symptoms, when your viral load is higher and the antigens pile up, those tests are closer to 80% accurate.

PCR tests are much more sensitive, but they can cost over $100 and insurance plans are no longer required to cover it. Plus it can take a couple days to get the results.

That means you might not be able to know for sure whether or not you have COVID on the day you start feeling sick. so you should still wear a mask and isolate from others to the best of your ability, say our experts.

And COVID isn't the only thing going around these days – the flu and RSV are still spreading across the U.S. Which is why virologist Jeremy Kamil emphasizes, "if you're sick, stay home."

CDC guidelines say if you get COVID, isolate for at least five days. And then, when your symptoms have gone away, they recommend testing negative two times in a row 48 hours apart on at-home tests before it's safe to unmask around others. CDC says that's because negative tests aren't always accurate. Is that two-test regimen really necessary?

We posed your question to the experts interviewed for this story: If you've tested positive and now you're feeling better and test negative, do you need a second test to confirm it?

Our experts agree: One negative test should be enough – but this only applies when you're recovering from COVID and not when you first feel sick (see previous answer for more on the vagaries of testing early on).

We reached out to the Centers for Disease Control and Prevention for response but they referred us to the FDA for comment about the recommendation for two tests to confirm that a patient no longer has COVID.

The FDA, which certifies the home antigen tests, only recommends repeat testing in the context of when you first get sick and deferred questions about masking and related topics to the CDC.

Addressing the matter of taking two tests at the end of your illness, virologist Jeremy Kamil noted, "I think that's a little overzealous in today's environment."

As for when to unmask when you're recovering from a confirmed case of COVID, our experts say it's safe to do so after a single properly performed negative test.

But they emphasized that it is important to think about others when making the decision to unmask and you may want to be extra cautious in certain environments. "Context is everything," says Kamil. If it's only been a day since you tested negative and you're going to be around people who are elderly, immunocompromised or unvaccinated, consider keeping your mask on for a while to minimize the risk of passing COVID to those people.

How long should I go between boosters? When a new variant emerges, is the current booster still effective?

Our experts say it's ideal to get boosted about every six months to keep your immunity at its highest. Even if you can't manage to get boosted that often, virologist Jeremy Kamil says that at a minimum you should aim to get boosted once a year.

And yes, a new variant might emerge that the vaccine isn't specifically designed to target. Professor of pharmacology Tim Brown says that which variant is circulating shouldn't play into your decision to get boosted.

In developing the updated booster vaccines, scientists try to predict what variants will be circulating in the next several months just as they do for the annual flu shot. But this process isn't perfect. For example, the spike protein variant used in the current boosters doesn't exactly match the spike protein of the newly emerged JN.1 variant.

But Brown says you should get the booster if you're due for one anyway. "Even though the vaccine may not be directly [designed for] the variant, the vaccine still helps your body protect itself against the infection. You may still get COVID but the symptoms will be less serious," he says.

And even though the current booster not being tailored for JN.1, Dr. Abraar Karan says there's data suggesting the vaccines help prevent long COVID and other severe symptoms – another reason to look to keep up with boosters.

One exception to this rule is if you've had COVID in the last three or four months. Then, Kamil says, "you might want to wait until you're closer to six months out [from being infected]," before getting boosted in order to get the fullest effects of vaccination. If your immune system is already primed from a recent case of COVID, the vaccine won't enhance your protection by much.

Max Barnhart is a Ph.D. candidate and science journalist studying the evolution of heat-stress resistance in sunflowers at the University of Georgia.

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Tags: antigen test  COVID boosters  rapid tests  COVID-19  immunity