Risk of Myocarditis after Covid-19 mRNA Vaccination: Impact of Booster Dose and Dosing Interval

Covid-19 mRNA vaccines have been shown to be associated with a short-term increased risk of myocarditis, with the highest risk observed after the second dose compared to the first. The extent of the risk associated with more distant booster doses is less clear. Here, we aimed to assess the relation between dosing interval and the risk of myocarditis, for both the two-dose primary series and the third dose (first booster). Extending our previous matched case-control study, we included 4 890 cases of myocarditis aged 12 or more and 48 900 controls up to January 31, 2022. We found that the risk of myocarditis remained elevated after the booster dose and that longer intervals between each consecutive dose (including booster doses) may decrease the occurrence of vaccine-associated myocarditis.