People who had side effects and reactions to the first COVID-19 vaccine dose might have had some level of the novel coronavirus in their system before they got their shot, Yahoo! Life reports.

What’s going on?

Health experts recognized that people who get their second shot of the COVID-19 vaccine tend to experience some side effects for a day or two. But there is a group who gets symptoms after the first dose, too.

Dr. Erin Morcomb, a physician at Wisconsin’s Mayo Clinic Health System, told Yahoo! Life that people who got their first dose experienced symptoms because they already had COVID-19 in their system.

  • “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 told Yahoo! Life.
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People who previously contracted COVID-19 have an immune system ready to fight it. So the first shot acts like a second shot for someone who didn’t get COVID-19.

  • “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.”

Why the first dose is effective

This also adds to the body of evidence that one dose might be enough for those who had COVID-19, at least in the beginning. As I wrote for the Deseret News, there’s research out that says the first dose of the Pfizer COVID-19 vaccine — which has two shots — can create high efficacy in fighting the novel coronavirus. In fact, the first dose of the Pfizer COVID-19 vaccine has been found to be 85% effective against COVID-19 symptoms after 15 to 28 days, The Wall Street Journal reports.

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That same study found it has 75% efficacy against stopping asymptomatic cases of COVID-19, too, per The Hill.

That said, experts still recommend people get both doses of the COVID-19 vaccine (or one shot of Johnson & Johnson) for full protection against the coronavirus.