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What Taking a COVID Booster Does to Your Body

Mild side effects are a good sign.

In the last six weeks, FDA has granted emergency use authorization to COVID booster shots by Pfizer-BioNTech, Moderna, and Johnson & Johnson. An estimated 70 million people are in the groups eligible for a booster. If that includes you, you might be wondering what to expect. Here's what getting a COVID booster shot does to your body. Read on to find out more—and to ensure your health and the health of others, don't miss these Sure Signs You May Have Already Had COVID.


It Reinforces Immune Response to COVID

woman getting over a cold

A COVID booster shot—given at least six months after the Pfizer or Moderna two-shot vaccines, or at least two months after the one-shot Johnson & Johnson vaccine—strengthens your immune system's response to the virus that causes COVID-19.

Like the first two shots, the COVID booster teaches the body to make the spike proteins found on the surface of the coronavirus, so the immune system can recognize it, fight it, and eliminate it. 

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It Boosts Your Antibodies

Antibodies attacking SARS-CoV-2 virus

Studies have found that if you originally got the Pfizer or Moderna shots, those companies' boosters increase the number of antibodies (the immune system's fighters that find and neutralize disease-causing pathogens) to COVID-19 by about 10 times.

One study found that people who got the Johnson & Johnson vaccine, then a Pfizer booster, saw their antibodies rise by 35 times. Johnson & Johnson recipients who got a Moderna booster saw their antibodies rise 76-fold

According to the company's data, getting a Johnson & Johnson booster shot after a first dose of that vaccine can increase your protection against symptomatic infection to 94%, up from 72%.

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It Doesn't Make You Invincible

woman covered with plaid checking her body temperature while sitting in bed at her apartment

Getting a COVID booster doesn't guarantee that you won't be infected by the coronavirus. It does, however, give you serious protection against severe illness, hospitalization or death from COVID-19.

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You May Have Side Effects Similiar to Your Earlier Shot


As with the initial vaccine doses, some people might experience mild side effects after getting the booster. These are a good sign—your immune system is learning how to fight off a coronavirus infection. (Some people don't experience side effects, and that doesn't mean the booster didn't work.) 

The FDA has published a list of the side effects most commonly experienced by clinical trial participants of Pfizer and Moderna boosters. They are similar and include pain, redness and swelling at the injection site, fatigue, headache, muscle or joint pain, and chills.

The FDA said that among people who got Moderna, swollen lymph nodes in the underarm were more common after the booster than after the initial two doses. Other than that, it hasn't been reported that side effects from booster shots are more severe than those experienced with initial doses.

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How to Stay Safe Out There

Brunette woman wearing a KN95 FPP2 mask.

Follow the fundamentals and help end this pandemic, no matter where you live—get vaccinated ASAP; if you live in an area with low vaccination rates, wear an N95 face mask, don't travel, social distance, avoid large crowds, don't go indoors with people you're not sheltering with (especially in bars), practice good hand hygiene, and to protect your life and the lives of others, don't visit any of these 35 Places You're Most Likely to Catch COVID.

Michael Martin
Michael Martin is a New York City-based writer and editor. Read more about Michael