Heart inflammation after vaccination with Covid-19 mRNA vaccines remains rare but teenage boys and young men are most at risk shortly after a second dose, a large study has confirmed.
A review of 46 studies covering more than 8,000 reported cases also found that most cases are mild but there may be a case for a longer interval between vaccines in some groups, Canadian researchers reported in the BMJ.
The research also showed that the risk of myocarditis and pericarditis may be higher after a Moderna rather than a Pfizer vaccine.
They also found the incidence of myocarditis in children aged 5 to 11 years was very rare but the data was limited.
Overall, rates of myocarditis after mRNA vaccines were 50-139 cases per million in 12-17-year-olds and 28-147 cases per million in 18-29-year-olds.
For girls and boys aged 5-11 years and females aged 18-29 years, rates of myocarditis after vaccination with Pfizer might be fewer than 20 cases per million, they reported.
Other findings include that for individuals aged 18-29 years, myocarditis is probably higher after vaccination with Moderna than with Pfizer, and for 12-39-year-olds, myocarditis or pericarditis might be lower when the second dose is given more than 30 days after the first dose.
Data specific to 18-29-year-old men suggested that the dosing interval might need to increase to more than 56 days to substantially drop rates of these conditions.
But they said the data suggesting that the risk may be lower if doses are given more than 30 days apart or in males aged 18-29 years more than 56 days apart was of low certainty.
In the short-term these side effects have consistently been shown to be quite mild and self-limiting but more data for longer term prognosis is needed.
They concluded: ‘As the Covid-19 pandemic enters its third year, continued surveillance of myocarditis after mRNA vaccines, especially in younger ages, after dose three (and subsequent doses) and in previous cases is needed to support continued decision making for Covid-19 boosters.”
Additional monitoring would be wise for people with relevant underlying conditions as well as long-term follow-up of patients with myocarditis, and further studies to better understanding of the mechanism underpinning the adverse effect, they added.
A linked editorial noted that some of the evidence was weak making it hard to draw firm conclusions.
‘Key uncertainties remain, including risks associated with boosters, risks associated with primary vaccination of young children, and the long term outcomes of those who experience myocarditis.
‘But these uncertainties must be placed in the context of the substantial and widely accepted benefits of vaccination,’ it said.