Coronavirus FAQ: What does it mean if the booster knocks me out? What if it doesn't?

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After I got my initial COVID-19 vaccine, my head hurt and my muscles ached. After I got my booster, I felt nothing. Please tell me it still worked!

You're in luck: Just as they did after the first round of shots, experts are quick to reassure that the vaccine works regardless of how you feel afterward.

"This is the first vaccine in history where anyone has ever complained about not having symptoms," Dr. Paul Offit, an immunologist and the director of the Vaccine Education Center at Children's Hospital of Philadelphia, told NPR in April.

If you don't have symptoms, "consider yourself lucky," Stanford infectious disease physician Abraar Karan said in February.

That holds true for the boosters — which is especially good news in light of preliminary data showing that the Pfizer-BioNTech booster appears to work about as well against the omicron variant of the coronavirus as earlier doses did against earlier variants.

So you're covered either way. If you do get side effects, "at least you know it's working," says Charlotte Baker, a professor of epidemiology at Virginia Tech. "But if you don't, I wouldn't say it's not working. Your body's response might not do anything outwardly."

That's because the vaccine provides protection in a few different ways. One of those ways is by triggering certain receptors on immune cells, and that can result in fatigue, headaches and other common side effects, as observed by a small University of Pennsylvania study. Those receptors don't work as well in older people, so side effects could be less noticeable in them. But their immune systems are responding in other ways — albeit more gradually.

The same holds true for boosters. Indeed, side effects of the booster are mimicking those of the initial doses, as you can see from the near-identical data that Pfizer submitted to the Food and Drug Administration after its studies: 61.5 percent of study participants developed fatigue, 54 percent had headaches and 39.3 percent dealt with muscle pain after getting Pfizer's second shot. After getting boosted, 63.7 percent of study participants experienced fatigue, 48.4 percent had headaches and 39.1 percent felt muscle pain. No new side effects or adverse reactions showed up. (Abnormal menstrual cycles and purple toe were among the lesser-known reactions on the first go-around.)

But there are a few differences.

Fewer people have reported fevers after the Pfizer booster: 16.4 percent of participants in the second-dose study reported fever symptoms, compared with 8.7 percent of those in the booster study. And those who got the Moderna booster reported fewer reactions overall than after the second dose of the original (although unlike Pfizer, Moderna reduced its dose for the booster).

People who mix and match brands, however, may be more likely to be down for the count. A study in The Lancet showed that more people who received a mix of the Pfizer and AstraZeneca vaccines reported fever, chills, headache, joint pain and muscle aches than those who received a booster of the same brand, Karan points out.

"What may be happening is that the road map your body had for the first one is a little different with a changed vaccine," Baker says. "The new vaccine has to teach it a few more things, so you may get new side effects. The vaccines all are trying to do the same thing, but each has their own way of doing it. And teaching your body new things — that's good."

The bonus of mixed brands is that early data shows that this approach may offer the best immune protection. But talk to your doctor about which might be best for you, Baker urges. "I chose Moderna because [there was evidence] it worked better in immunocompromised people after Johnson & Johnson," she says.

If you do have any concerns about anything related to your vaccine, either about side effects or how the vaccine was administered, you can report those concerns to the Centers for Disease Control and Prevention via a text-based service called v-safe, Baker notes. After you enter your info, you'll get a message stating, "Depending on your answers, someone from CDC may call to check on you."

The bottom line: "Getting the booster has been proven to significantly reduce your risk of contracting SARS-CoV-2 compared to only getting a primary vaccine series," Karan says. "Because of omicron especially, we would urge that people get boosted, avoid crowded areas and try to wear high-grade masks if they have to be in a crowded public area."

And with school breaks coming up accompanied by holiday plans, this is a perfect time to schedule those shots, Baker adds. With any luck, you'll be able to arrange it so potential side effects don't dampen your holiday fun.

Sheila Mulrooney Eldred is a freelance health journalist in Minneapolis. She has written about COVID-19 for many publications, including The New York Times, Kaiser Health News, Medscape and The Washington Post. More at sheilaeldred.pressfolios.com. On Twitter: @milepostmedia.

Copyright 2021 NPR. To see more, visit https://www.npr.org.

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