Quick note: No Q&A this weekend; back to my writing project. And since some of you asked, I committed to writing a book chapter and have to fulfill my promise. Q&A will likely be paused over weekends for the next month or so. 

Question: What are the  mechanisms by which someone might develop “COVID arm” 10 days after receiving the shot and with no prior symptoms?  I am asking because I developed COVID arm, and thought my body was gaslighting me until I googled it and found one news article that said that a rash 10 days later was a thing — mine was just around the injection site, but with significant swelling, 8 days after the shot.

Answer:  You are not alone in your experience.  COVID arm is indeed a thing, and it’s nothing to be alarmed by!  While it’s not that common,* it is happening enough to have made the news, been incorporated into some CDC guidance, been written about in the scientific literature.  For readers who are wondering what we’re talking about, “COVID arm” is a delayed reaction associated with the Moderna and Pfizer mRNA vaccines that can include a localized rash, swelling, pain and itching on/around the vaccine injection site.  Earlier this month, the New England Journal of Medicine published a comment describing the phenomena, Delayed Large Local Reactions to mRNA-1273 Vaccine against SARS-CoV-2.  Authors note that the delayed reaction generally occurs 8 days after injection (range: 4-11 days) and resolves within 4-5 days.  Furthermore, the authors propose that “T-cell–mediated hypersensitivity” is the biological mechanism causing the delayed reaction and present some compelling data supporting this hypothesis.  

Now you may be wondering, what is “T-cell-mediated hypersensitivity”?  Basically, it’s an allergic reaction caused by an intense immune reaction stemming from T-cells (likely CD4 “helper” cells).  Since it takes several days for the adaptive immune system to kick in and for T-cells to mature, the reaction is generally delayed.  We still don’t know why some people experience such hypersensitivity; scientists are exploring the factors/mechanisms.  In good news, when it comes to COVID-arm, there is nothing to be overly concerned about– though uncomfortable and even painful, it will resolve and it will not cause lasting harm– and experts encourage and recommend that individuals experiencing COVID-arm take the second dose. In terms of treatment, CDC recommends “If the rash is itchy, you can take an antihistamine. If it is painful, you can take a pain medication like acetaminophen or a non-steroidal anti-inflammatory drug (NSAID).”

*In the Moderna clinical trial, 0.8% of participants experienced a delayed reaction after the first dose, and 0.2% experienced delayed reaction after the second dose.