Rare heart-related side effects higher with Moderna COVID vaccine

Though both complications were rare, data from Ontario show higher rates of myocarditis and pericarditis with the Moderna COVID-19 vaccine than with the Pfizer-BioNTech vaccine, but the rates were lower for both vaccines if the spacing between receiving two doses was extended, according to a study late last week in JAMA Network Open.Myocarditis is an inflammation of the heart muscle, and pericarditis involves swelling of the thin membrane around the heart.Five times higher rate with short dosing intervalFor the study, researchers from Public Health Ontario, the University of Toronto, and the nonprofit ICES analyzed data on 19,740,741 doses of mRNA COVID vaccine administered in the province from Dec 14, 2020—when vaccines were first available—to Sep 4, 2021.Ontario's electronic reporting system revealed 297 cases of myocarditis or pericarditis among those vaccine recipients, or only about 15 cases per million vaccine doses.Of the 297 cases, 228 (76.8%) occurred in male recipients, and the median age of affected patients was 24 years (range, 12 to 81 years).

Also, 207 (69.7%) of the cases occurred following the second vaccine dose.Nearly all of the cases (290, or 97.6%) involved an emergency department visit, with 210 cases (70.7%) also requiring hospital admission.

The number of hospitalizations was 87 (82.9%) for myocarditis, 33 (38.8%) for pericarditis, and 90 (84.1%) for people who had both conditions.When the investigators focused solely on people who received their second dose during the period of enhanced passive surveillance (from Jun 1, 2021, onward), they found that the incidence of myocarditis or pericarditis was 62.5 cases per million doses after the second Moderna dose (95% confidence interval [CI], 42.4 to