Both Moderna and Pfizer followed up with volunteers at least two months after they received their second injection (both vaccines require two doses). No major safety concerns were found.
“Documented adverse effects to vaccines show up mostly in hours, sometimes in days, rarely in weeks,” Megan Ranney, director of the Brown University Lifespan Center for Digital Health, recently wrote on Twitter.
With mRNA vaccines in particular, the body destroys the molecule fairly quickly, meaning it won’t linger in the body.
Most Pfizer participants reported temporary side effects after being vaccinated, including fatigue, headaches, and pain at the injection site. People under 55 reported more side effects overall. Among people aged 18 to 55, 4.6% reported severe fatigue after getting their second injection, and 3.2% had severe headaches.
In Moderna’s trial, nine in 10 people who got the shots registered some side effects, but they were mostly mild or moderate. Around 17% reported more serious side effects including pain or swelling at the injection site, fever, headache, fatigue, nausea, and vomiting. The side effects typically lasted for one to three days, according to documents released Tuesday by the FDA.
In both trials, these results were consistent across genders, age groups, ethnic and racial groups, and people with preexisting medical problems.
“Prior experience shows that after a vaccine has been approved by the FDA, it is rare to discover any significant new serious unexpected side effects,” Dr. Noam Tau, a physician at Sheba Medical Center, said in a statement to Business Insider.