TAMPA, Fla. — Health experts say the reason why more people are having reactions to COVID-19 vaccines compared to flu vaccines is because they're made differently.
“The Pfizer and Moderna vaccines have the two shots, so the second dose is usually worse,” said Dr. Michael Teng, a virologist and associate professor for USF Health.
Experts say the reactions are typically mild to moderate symptoms. The most common side effects are fever, chills, headache, and fatigue.
Doctors say those reactions are a good thing.
“It really means that your immune system is sort of ramping up," Teng said. "It’s your immune system causing those side effects."
Side effects of the Johnson & Johnson vaccine will likely present within a few hours of getting vaccinated.
For the two-shot Moderna and Pfizer vaccines, doctors say after the first dose, a lot of people won’t typically have too many side effects.
“The first time you get the vaccine, it’s just training your immune system to recognize this one protein from the coronavirus. The second time around, your immune system is already primed, already has seen that, already recognizes it," Teng said. "So what you’re doing there is not to trying to train it again. What you’re trying to do is make it get better — get better at recognizing the protein."
It’s only a few hours after the second shot when people will usually feel more symptomatic.
“Now it’s primed, it’s going to respond quicker and it’s going to respond more robustly, so you’re going to have more symptoms,” Teng said.
Some experts speculate people who have more intense reactions after the first shot of the Pfizer or Moderna vaccine likely had COVID-19 previously.
“We know that the people who have tested positive, who have known COVID exposure or prior COVID exposure, when they get the first dose of Pfizer or Moderna vaccine, they have significant side effects," Teng said. "It looks almost as if they’ve gotten the second dose, and that’s because their immune system has been primed from the initial reaction.”
“One thing that we don’t know is how many of these people that are experiencing significant side effects the first time, were they in fact exposed prior to vaccination and they had an asymptomatic infection that was so low grade they didn’t even know it, so that might be part of it as well,” Teng said.
This story was originally published by Larissa Scott on WFTS in Tampa, Florida.
