ATLANTA — The U.S. Centers for Disease Control and Prevention has updated its advice to say males between 12 and 39 might consider waiting eight weeks between first and second doses of Pfizer and Moderna’s vaccines to reduce still-rare risk of heart inflammation.


The increase on the minimum approved interval of three and four weeks respectively comes in the wake of several studies linking those vaccines to increases in rates of pericarditis, inflammation of the outer lining of the heart, and myocarditis, inflammation of the heart muscle.


While incidence of COVID-19-associated cardiac injury or myocarditis are estimated to be 100 times higher than those related to vaccines, and the latters’ symptoms are usually far milder and resolved within weeks, a Nature Reviews Cardiology study suggests one possible mechanism behind the issue is that the immune system might detect the messenger RNA in the vaccines as an antigen, resulting in “the activation of pro-inflammatory cascades and immunological pathways in the heart.[c]”


As so-called mRNA vaccines, both Pfizer and Moderna’s vaccines package messenger RNA, or the genetic sequence of the novel coronavirus, into a lipid nanoparticle carrier in order to program cells to reproduce the coronavirus spike protein so that it can later be recognized by the body’s immune system should it come into contact with the actual virus.


For now, though, there are several active theories about how this process could trigger heart inflammation, with the increased incidence among male patients also suggesting hormone signaling might be involved.