Pfizer Caused This Reaction in Half of Recipients, New Study Says

This is the second most common side effect from Pfizer's COVID vaccine.

There's been a lot of talk about vaccine side effects, and that makes sense: When you do get your COVID shot, you want to know what's coming your way in the ensuing hours. To help break things down, one recent study looked at all of the most common side effects of the Pfizer and Moderna vaccines. The report found that there was one vaccine reaction that nearly half of the people who got the Pfizer shot had after the second dose. Read on to find out which side effect you have a 50-50 chance of experiencing with Pfizer, and for a look at the most common Moderna side effect, Moderna Caused This Reaction in 82 Percent of People, New Study Says.

Almost half of Pfizer recipients experienced fatigue after the second dose.

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A study published by the Journal of the American Medical Association (JAMA) on April 5 looked at reports from almost two million people who received both doses of the Pfizer or Moderna vaccine. The reports were collected through a Centers for Disease Control and Prevention (CDC) program called v-safe that tracks vaccine side effects. The data showed that almost half of all Pfizer recipients—48 percent—experienced fatigue after the second dose.

Fatigue was much more common after the second dose of the vaccine, with just 29 percent of Pfizer recipients reporting it after the first. This was the most common systemic side effect of the Pfizer vaccine. However, it was even more common among Moderna recipients, with 32 percent and 60 percent of those recipients experiencing fatigue after the first and second dose, respectively. And to make sure you're recovering properly, You Need This in Your Diet After Your COVID Vaccine, Doctor Warns.

Pain at the injection site was the most common side effect of Pfizer.

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Injection site reactions are common among vaccines and nothing to worry about, according to experts. The study found that 64 and 67 percent of Pfizer recipients reported pain at the injection site after the first and second dose, respectively. Other common injection site reactions following a Pfizer shot included redness, swelling, and itching. The researchers noted that "injection site pain is common after both the first and second doses of either mRNA-based vaccine." And for more reactions to expect, Be Prepared for This the Night You Get Your COVID Vaccine, Doctors Warn.

Other frequently recported Pfizer side effects included headache and muscle pain.

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Everyone's response to the COVID vaccine is different, but this study found that there were a few side effects that occur often in Pfizer recipients. After pain at the injection site and fatigue, the most common side effects of Pfizer included headache, muscle pain, chills, and fever. After the first dose of the shot, 25 percent of Pfizer recipients reported a headache, and 40 percent reported one after the second dose. Muscle pain was reported by 17 percent of Pfizer recipients after the first dose, and 37 percent after the second. And for more COVID vaccine news delivered straight to your inbox, sign up for our daily newsletter.

The Pfizer vaccine has a lower rate of reported side effects than the Moderna vaccine.

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While both the Pfizer and Moderna vaccines are highly safe and effective, the study found that there was a notable disparity between the vaccines when it came to side effects. According to the study, more Moderna recipients reported side effects in every single category. "A greater percentage of participants who received the Moderna vaccine, compared with the Pfizer-BioNTech vaccine, reported reactogenicity [reactions that occur soon after vaccination]; this pattern was more pronounced after the second dose," the study reads. And for one of the odder vaccine reactions, discover The Strange New COVID Vaccine Side Effect That's Confusing Even Doctors.

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