Side Effects of COVID-19 Vaccine

After you receive the COVID-19 vaccines by Pfizer or Moderna, you might experience some minor, brief side effects similar to the flu: Fever, chills, body aches.

Here’s why: “Both Pfizer and Moderna are considered ‘reactogenic’ in that they stimulate a strong immune response with unpleasant but temporary side effects,” explains Dr. George Hwang, an anesthesiologist in Arlington, Virginia, who recently experienced side effects himself after receiving the vaccine — what he calls “the happiest headache, fever and chills I ever had.”

That’s because he knew they were a sign his body had begun developing immunity to COVID-19. Here are the seven most frequently reported post-shot side effects for the Pfizer and Moderna vaccines, from least common to most common.

1. Least Common Side Effect: Fever!

After the vaccine is injected into your arm, your cells take in mRNA that encodes a version of the coronavirus’s spike protein. And the body reacts. “The innate immune system sets off alarms to recruit more immune cells to your arm, causing inflammation and soreness, which then results in more immune cell recruitment causing the bigger picture symptoms of fever…