Erin Kissane, a co-founder of the COVID Tracking Project, rolled up her sleeve for the Novavax COVID-19 vaccine in mid-October soon after it was finally recommended in the United States. Like many people with autoimmune diseases, she wants to protect herself from a potentially devastating COVID infection.

Kissane's autoimmune arthritis seems to make her susceptible to unusual vaccine side effects. After getting an mRNA booster last year, her joints ached so painfully that her doctor prescribed steroids to dampen the inflammation. She still considers the mRNA vaccines "miraculous," knowing COVID could be far worse than temporary aches.

Nonetheless, when the pain subsided, she pored through studies on Novavax's shot, a vaccine that is based on proteins rather than mRNA and has been used since early 2022 in other countries. Data from the United Kingdom found that people more frequently reported temporary reactions — like low fevers, fatigue, and pain — as their immune system ramped up in the days following booster vaccination with Moderna's mRNA vaccine versus the one by Pfizer. And those boosted with Novavax's had fewer complaints than either of those. That finding was corroborated in an analysis of international data published last year.