Lack of side effects doesn't mean mRNA vaccine not working

While a variety of side effects after receiving an mRNA COVID-19 vaccine may be a sign of the immune system kicking into high gear, a lack of such reactions does not mean it has failed to respond, researchers have found.

They tested 206 hospital employees for antibodies against the coronavirus before and after receipt of the vaccine from Pfizer and BioNTech and surveyed them about vaccine-related reactions. As in clinical trials, arm pain was the most common symptom, reported by 91% after the first shot and 82% after the second.

Systemic symptoms, such as feeling weak or tired, or having body aches or pains, were reported by 42% and 28%, respectively, after the first shot and by 62% and 52% after the second shot.

But there was no correlation between vaccine symptom severity and antibody levels one month after vaccination, the researchers reported on Friday in a paper posted on medRxiv ahead of peer review.

The researchers said the findings should reassure people that a lack of side effects after getting the mRNA shots does not mean the vaccine is not working as intended. The Moderna shot also uses mRNA technology.