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Ask An Expert: Why Does The COVID-19 Vaccine Sometimes Cause A Fever?

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Vita Susin, a respiratory therapist at UC Irvine Health, prepares to be vaccinated against COVID-19. Chava Sanchez/LAist
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Some people feel mildly sick after getting the COVID-19 vaccine although the shot doesn't contain the live virus.

We asked Doctor Kimberly Shriner, an infectious disease specialist at Huntington Hospital in Pasadena, why that happens.

"When you get vaccinated, the vaccine introduces the recipe for the spiked protein on the virus. It teaches the cell to make only the spiked protein, not to make the whole virus. That's why you don't get COVID," Shriner says.

Shiner explains that the spiked protein shows the immune system how to make antibodies for COVID-19.

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That's why in the first day or two after getting the first or second shot, some people feel a bit draggy or have a little fever, Shriner says. Having a fever can be a sign that your body is doing its job and creating antibodies.

Sometimes the second shot can cause more side effects.

"Your immune system now knows that spiked protein and goes, 'Ooh, I've seen that before and I don't like it. I'm going to make some antibodies,'" Shriner says.

Those side effects you might be feeling? The fever and drowsiness are responses from your immune system — not the coronavirus.

Shriner says this doesn't mean that people who don't feel sick after getting the vaccine aren't having a good immune response. It only means some people are more sensitive than others.

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Note that fever and fatigue are mild to moderate side effects of the Pfizer and Moderna vaccines. An FDA analysis found "no specific safety concerns."

READ MORE ABOUT HOW THE COVID-19 VACCINES WORK:

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