What Are the Side Effects of Booster Shots? 

AARP shared that millions of Americans who originally received the Pfizer-BioNTech coronavirus vaccine series are now eligible for a booster dose of the same brand.

So much about this third shot, which is meant to rev up the immune system so that it stays sharp in the fight against COVID-19, will be just like the previous two. Pfizer’s third booster dose is the same formulation and the same strength as shots one and two. And data collected to this point suggests the side effects brought on by the booster are like the symptoms some people experienced after the initial set — even milder.

Here’s what we know so far about the side effects of Pfizer’s booster shot.

Booster trial reveals no new surprises

Pain at the injection site was the most commonly reported reaction after receiving the booster, according to the clinical trial data Pfizer and BioNTech submitted to the U.S. Food and Drug Administration (FDA). About 83 percent of the trial’s booster recipients reported it, followed by fatigue (63.7 percent) and headache (48.4 percent), most of which were mild to moderate. These findings closely mirror the side effect data collected from Pfizer’s second vaccine shot. Other side effects recorded in the booster trial also fall in line with symptoms documented after the primary Pfizer series. They include muscle and joint pain, chills, diarrhea, vomiting and fever. And compared to adults ages 18 to 55, Pfizer’s trial found that adults 65-plus were less likely to experience these fatigue or flu-like symptoms after receiving the booster. 

That’s not surprising, says Melanie Swift, M.D., co-chair of Mayo Clinic’s COVID-19 Vaccine Allocation and Distribution Workgroup. Most of the side effects are not a result of the vaccine, directly, but rather “an indication of your immune system reacting” to the vaccine, she notes. Meaning, the more robust your immune response is, “the more side effects you’re going to have.”

But as people age, their immune responses typically dwindle, Swift says. This is why older adults are at the top of the list of people who should get a booster. New research suggests that while the vaccines continue to provide a strong defense against hospitalization and death from COVID-19, that protection wanes faster over time in older adults.

There were no reported cases of myocardium, pericarditis, anaphylaxis, appendicitis, or Bell’s palsy in the booster trial population (around 300 adults) during the study period. One symptom that the FDA highlighted, however: Swollen lymph nodes in the underarm, were observed more frequently following the booster dose than after the primary two-dose series.


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