The overall risk of myopericarditis -- a condition that causes inflammation of the heart muscles -- following COVID-19 vaccination is very low, affecting 18 people per million vaccine doses, according to a study published in The Lancet Respiratory Medicine journal.

The research confirms that the risk of myopericarditis is comparable to or lower following COVID-19 vaccination than other non-COVID-19 vaccines. The team analysed international databases, looking at over 400 million vaccination doses, to compare the risk of myopericarditis following vaccination against COVID-19 and other diseases such as influenza and smallpox.

They found no statistically significant difference between the incidence of myopericarditis following COVID-19 vaccination -- 18 cases per million doses -- and other vaccinations, which was 56 cases per million doses.

"Our research suggests that the overall risk of myopericarditis appears to be no different for this newly approved group of vaccines against COVID- 19, compared to vaccines against other diseases," said Kollengode Ramanathan, from National University Hospital, Singapore, and corresponding author of the study.

"The risk of such rare events should be balanced against the risk of myopericarditis from infection and these findings should bolster public confidence in the safety of COVID-19 vaccinations," Ramanathan said. Myopericarditis, in some cases, may cause severe permanent heart damage. It is most often caused by viruses but can also occur after vaccination in rare instances. There have been reports of myopericarditis following mRNA-based COVID-19 vaccination, especially in adolescents and young adults.

The researchers analysed more than 20 studies with reported incidences of myopericarditis following any type of vaccination between January 1947 and December 2021. Among COVID-19 vaccinations, the risk of myopericarditis was higher for those who received mRNA vaccines (22.6 cases per million doses) compared to non-mRNA vaccines (7.9 cases per million doses), the researchers said.

Reported cases were also higher in people below the age of 30 (40.9 cases per million doses), males (23 cases per million doses), and following the second dose of COVID-19 vaccine (31.1 cases per million doses), they said.

"The occurrence of myopericarditis following non-COVID-19 vaccination could suggest that myopericarditis is a side effect of the inflammatory processes induced by any vaccination and is not unique to the SARS-CoV-2 spike proteins in COVID-19 vaccines or infection", said Jyoti Somani, from National University Hospital, Singapore, and co-author of the study.

"This also highlights that the risks of such infrequent adverse events should be offset by the benefits of vaccination, which include a lower risk of infection, hospitalisation, severe disease, and death from COVID-19," Somani said. The researchers acknowledge some limitations of their study.

They noted that their findings include only a small proportion of children under the age of 12 who have only recently been eligible for vaccination and that the results of this study cannot be generalised to this age group.