This Side Effect Is More Common After Your First Pfizer Dose, Study Says

You're more likely to experience this reaction after your first shot than your second.

Side effects are common after many shots, but we've all been paying closer attention to them than ever with the COVID vaccine. For many, side effects have been more intense after the second dose than the first, whether it's the Moderna of Pfizer vaccine. However, a new study has found that one side effect is more common after the first Pfizer dose than the second. Read on to find out what reaction you should be anticipating after your first shot of the Pfizer vaccine, and for more on this specific vaccine, Pfizer's Vaccine Protects You for at Least This Long, Study Finds.

Arm tenderness was more commonly reported after the first Pfizer dose than the second.

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A new study published in the journal Lancet Infectious Diseases on April 27 looked at the side effects patients experienced after receiving the Pfizer vaccine between December and March in the U.K. According to the study, one side effect was actually more commonly reported after the first Pfizer dose than the second. Out of 655,590 people who reported vaccine side effects, the researchers found that 57.2 percent of Pfizer recipients reported experiencing arm tenderness after receiving the first dose. However, that number went down to 50.9 percent after the second dose. And for more on vaccine reactions, This Vaccine Side Effect Could Mean You Already Had COVID, New Study Says.

It was also the most common side effect reported in general after either dose.

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Arm tenderness was the most common side effect reported in general among those who got the Pfizer vaccine. The second most common side effect was arm pain, which 29.2 percent reported after the first dose and 34.3 percent reported following the second.

The most common systemic side effects, meaning those that affect your body beyond the injection site, were fatigue and headache, the study found. More than 8 percent of Pfizer recipients reported feeling fatigue after their first Pfizer dose and 14.4 percent reported it after their second. And 7.8 percent reported a headache after their first shot, with 13.2 percent reporting the side effect after their second dose. And for more up-to-date COVID vaccine information, sign up for our daily newsletter.

All other side effects were more common after the second dose of Pfizer.

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Arm tenderness was the only symptom more commonly reported after the first dose of Pfizer. In general, systemic side effects were significantly more common after the second dose. According to the study, 22 percent of patients experienced systemic reactions after their second Pfizer shot, but only 13.5 percent reported the same after the first dose.

For all specific side effects reported besides arm tenderness—chills, diarrhea, fever, joint pain, muscle pain, nausea, arm pain or swelling, itching, swollen armpit glands, and warmth, redness, or bruising on the arm—more patients reported experiencing them after the second dose. For instance, 6.4 percent reported experiencing chills after their second shot of Pfizer, but only 2.5 percent reported experiencing them after their first. And for more reactions to be prepared for, The Common Vaccine Side Effect That No One Is Talking About, Experts Say.

There's a reason side effects are typically more intense after your second dose.

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According to the Centers for Disease Control and Prevention (CDC), "side effects after your second shot may be more intense than the ones you experienced after your first shot." The reason is because the first shot introduces your body to how the virus behaves, while the second prepares to attack it.

"The second is really like a booster dose," Bill Moss, MD, a professor of infectious disease epidemiology at the Johns Hopkins Bloomberg School of Public Health, told NBC News. "The immune system is seeing the vaccine for the first time with the first dose and is reacting to that. … So when the body's immune system sees [the vaccine] a second time, there are more cells and there's a more intense immune response, resulting in those side effects."

The CDC says arm tenderness is normal after either dose and likely to go away in a few days, but you should contact your doctor or healthcare provider if yours worsens after 24 hours. And for more on future shots, Pfizer's CEO Just Said How Often You'll Need a COVID Vaccine.

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