Coronavirus (booster) FAQ: Can it cause a positive test? When should you get it?

We regularly answer frequently asked questions about life during the coronavirus crisis. 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: "Weekly Coronavirus Questions." See an archive of our FAQs here.

A few weeks ago, my friend Ashley was preparing to take her final examinations for her Ph.D. and trying to take all the right precautions so that the exams would go smoothly.

"It was a Thursday afternoon. I had gotten the bivalent vaccine and the flu shot," she said. "Then the next day I started feeling bad."

At first, she attributed her symptoms to the effects of the COVID vaccine. But after a few days, she describes having a moment of uncertainty about the cause of her symptoms. "I was doing progressively worse, so on Saturday I was like, 'Oh, I have a fever and my arm is sore, let me rule out COVID just to make me feel better.'"

She took an at-home test.

"And it was positive..."

Which at first led her to Google – and then to a phone call to the university health center – all to answer the question...

Can you test positive for COVID from getting the vaccine?

Whoever answered the phone at the health center didn't seem to know the answer – and the Google results can be a bit confusing, so we decided to ask the experts.

"It would be absolutely impossible" to test positive because you got the vaccine, says Jeremy Kamil, a virologist at Louisiana State University Health Shreveport.

This is because the tests are "testing for something not in the vaccine," says Dr. Seth Cohen, an infectious disease physician and medical director of Infection Prevention & Control at the University of Washington Medical Center.

The vaccine and the tests are based upon different parts of the COVID virus. "All the vaccines in the United States use the spike genetic material," says Kamil. "All of the home tests detect something called nucleocapsid," a protein not found in the vaccine.

But if the vaccine is going to make you feel ill, then how can you tell whether you're sick from the vaccine or from a COVID infection?

The answer to that question has to do with the timing and nature of your symptoms. As for how long your vaccine symptoms might last, Cohen says "everybody's a little bit different," but you should start to be concerned if you have a fever, body aches or headaches that last more than 24 hours.

Cohen also says one symptom in particular is highly indicative of a COVID infection and not the vaccine. "If you have any respiratory symptoms, I would definitely get tested. The vaccine should not give you respiratory symptoms," he says.

So why did Ashley test positive so soon after getting the vaccine?

"Almost certainly what happened was she had been exposed to COVID prior to her vaccination, or maybe even the same day she was vaccinated," says Kamil. It's especially unfortunate because "the vaccine cannot protect you right away," Kamil goes on to explain, meaning getting the booster didn't help Ashley fight off her infection. He also said that it takes "three to four days" after getting the vaccine for your body to start creating antibodies and longer to develop full protection.

So no, the vaccine can't make you test positive. And for those wondering, Ashley was lucky enough to develop only mild symptoms. She successfully passed her Ph.D. defense safely over Zoom later that week.

"I didn't get to celebrate in person and had to do everything from my living room, which was a bummer, but I passed and it wasn't bad."

So congratulations, Ashley!

Does it matter which version of the booster shot I take? Is one of them better than the other?

So you've made the decision to get the new bivalent booster and make an appointment at the nearest pharmacy. But as you're filling out your personal information, there's a chance you'll be presented with a choice – Pfizer or Moderna? Which shot should you get?

The two brands of vaccine are supposed to do the same thing – protect you against the omicron variants of COVID – but are they really the same? Could one be better than the other?

"The Moderna shot has a tiny bit more RNA [the active part of the vaccine] in it than the Pfizer one," says Kamil. "However, I don't think there's any substantial difference in the protection you get."

Cohen echoes that point and adds that there shouldn't be a difference in the reactions you get from the vaccines. "The side effect profiles are very similar. The Moderna vaccine is a slightly higher dose, but we haven't necessarily seen more adverse reactions related to that."

And what about mixing brands? If you've gotten Pfizer so far, can you now get Moderna or vice versa?

"It's completely fine to mix brands," Cohen says. "Don't overthink it and don't waste an opportunity to vaccinate. Go with whichever vaccine your local pharmacy or clinic has."

Kamil says what's much more important than the brand of vaccine you get is when you get it. "If you only waited two months since your last booster [or since you were infected], you're gonna get less of a benefit than someone who waited six months," he says.

Which leads to the next question.

The CDC recommends waiting at least 90 days since a previous COVID infection before I get the new booster. But is it bad or harmful in any way if I get the new shot sooner?

While Kamil says waiting the appropriate amount of time will give you a better immune response, Cohen says getting a shot sooner "is not harmful or bad in any way."

Both Cohen and Kamil say that experts are predicting a surge in cases as the weather changes and the holidays approach due to increased indoor social activity. With that in mind, Cohen says "it doesn't make sense to necessarily skip or delay a vaccination when you might be heading into a surge."

As Thanksgiving approaches, Kamil says you might want to get your shot "in the middle or third week of October" if you got COVID or another booster in or before August.

I'm testing positive on one brand of self-test but not on another. Why is this happening and what should I do?

Maybe you're just starting to feel sick – or maybe it's been over a week since you first tested positive – but either way you're worried about having COVID and infecting other people.

So you take an at-home test, but you've picked up an assortment of brands over the past few months and a curious thing happens.

You test positive on one brand but negative on another.

Cohen says that "most of the major tests that are on the market are fairly comparable" in terms of their sensitivity to detecting the virus. But he also says that "there's more variability" in sensitivity for some of the lesser-known brands. Those lesser-known brand tests might not pick up the remnants of virus from a waning infection. But ultimately, Cohen says it's important to pick a brand of test "that feels intuitive. If it's too many steps or too complicated, it's not going to be a good at-home test for you."

We know from conversations with experts in our previous FAQs that a positive test means it's highly likely that you do have some COVID virus circulating in your body.

So if you're testing positive for the first time, get ready to isolate and mask up according to CDC guidelines. But if its been 10 days since you first tested positive and you're trying to figure out if you're no longer infectious, Cohen says to "take a [home] test, see if it's negative, and repeat it [in] 48 hours to confirm." That way you can be sure you're no longer infectious.