This study provided in vivo evidence that inadvertent intravenous injection of COVID-19 mRNA vaccines may induce myopericarditis. Brief withdrawal of syringe plunger to exclude blood aspiration may be one possible way to reduce such risk.

Preprint: Intravenous Injection of Coronavirus Disease 2019 (COVID-19) mRNA Vaccine Can Induce Acute Myopericarditis in Mouse Model

See also: “The middle of the vastus lateralis is an appropriate site for intramuscular injections because of the low risk of vascular or nerve damage.”

Anatomically safe sites for intramuscular injections: a cross-sectional study on young adults and cadavers with a focus on the thigh

Denmark’s SSI recommends changes to syringe injection method for coronavirus vaccines