As the coronavirus vaccines continue to be distributed across the country, there have been anecdotal reports of people experiencing more side effects after the second dose, but experts say this is a "normal immune response."
Andrew Heinrich, a lecturer at the Yale School Of Public Health in New Haven, Connecticut, said that so far the reports of more severe side effects are "anecdotal," so there is nothing "statistically valid about that trend." However, as more people receive the shot, he said it's something that people are "aware of and starting to ask questions about."
TODAY's Al Roker even mentioned his concerns about dealing with related side effects before he got his second dose of the Pfizer vaccine, noting in early February that he had been hearing stories from friends that had felt more symptoms after getting the second shot.
NBC News medical contributor Dr. Kavita Patel told Al that there's no reason to worry: While about a third of the people who receive the vaccine have symptoms that "seem like the flu," that's an expected immune response.
"The second vaccine (dose) — think of it as having that hit to your immune system, and your immune system now recognizes the vaccine, so it does its job," explained Patel, who said that she herself had had a reaction to the second dose. "... I felt, for about 36 hours, like I had the flu."
Dr. Bill Moss, a pediatrician and professor of infectious disease epidemiology at the Johns Hopkins Bloomberg School of Public Health in Baltimore, agreed with Patel's assessment.
"The second dose is really like a booster dose," he said. "The immune system is seeing the vaccine for the first time with the first dose and is reacting to that, and the cells of the immune system are recruited to kind of recognize that spike protein (the part of the coronavirus that the vaccine affects). 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."
Heinrich called the reaction a "normal immune response."
What side effects can people expect?
Side effects include localized reactions, like swelling, rash or soreness at the injection site, which have been reported in about 84% of recipients, according to Patel. About 63% of people have reported feeling some fatigue. The Centers for Disease Control and Prevention (CDC) note some people may also experience chills, fever or a headache, and suggest that people who are not feeling better after around 24 hours reach out to their doctor.
"Just be prepared," said Patel. "If you don't have a reaction, you don't need to worry that it didn't work. Every human and body is different."
CORRECTION (March 26, 10:30 AM): An earlier version of this article misstated Andrew Heinrich’s title as a professor at the Yale School of Public Health. He is a lecturer at the Yale School of Public Health, not a professor.