With over 40 million Americans now fully vaccinated against COVID-19, it's no secret that the shots can lead to unpleasant side effects such as fever, headache, body aches and fatigue. But while initial research suggested that individuals were more likely to experience these symptoms after the second dose, experts now say that those who previously had COVID-19 — whether knowingly or not — may end up reacting more strongly to the first dose.

One study released in early February from Bar-Ilan University in Israel found that the immune response created after the first dose of vaccine in those who previously had COVID-19 was "so effective" that it "opens the debate as to whether one dose of the vaccine may suffice." Another study published in the Lancet found a 140-fold increase in antibodies after the first vaccine among those who had previously had a COVID-19 infection versus those who hadn't.

Dr. Erin Morcomb, a family medicine physician at the Mayo Clinic Health System in La Crosse, Wis., and head of its COVID-19 vaccination team, confirms that the reactions can vary based on your health history. "What we've seen in studies is that the second dose does tend to have a little bit more potential to cause side effects than the first dose, but for people who have had COVID-19 infection previously and then recovered, they are at higher risk of having those same side effects after their first dose," she says.

The reason, she explains, is that those who developed the infection previously have an immune system that is already primed to fight it off. "After they've had their COVID-19 active infection, they've made some antibodies themselves in their body to the national infection," Morcomb says. "Then when they get their first dose, their body is already recognizing that they have some antibodies and they can make a really robust immune response to that first dose of vaccine."

She notes that those who had treatment with monoclonal antibodies need to wait 90 days before getting the vaccine as they can "block the immune system for making a good response to the vaccine."

For those who haven't had COVID-19, the first shot of either the Moderna or Pfizer vaccine ends up serving as the body's introduction to the virus. That's why the second dose, in those who haven't had COVID-19, can often cause a big reaction. "This time, the body recognizes it like, 'Hey, I've seen this before,'" says Morcomb. "And then it really tries to ramp up the immune response."

To be sure, that's not to say that those who don't have side effects should be concerned. Morcomb says there is no research suggesting that the absence of side effects is an indicator that the vaccine isn't working. "For some reason [some individuals], their bodies just don't show the side effects as much," she says. "So it's not like an absolute rule that just because you had COVID, you're going to have side effects, because it doesn't happen in everybody. And those people should still be reassured that they're making a good immune response to the vaccine."

As for how soon after a COVID-19 infection individuals can get vaccinated, she says there's good news there as well. "Initially we had recommended waiting 90 days from a COVID infection to get the vaccine but that actually isn't necessary — there's not a decreased effectiveness of the vaccine if you get it before that," she says. "So here we are recommending that once people come off of their quarantine — so 14 days — that they get their shot then."