Research shows that the risk of developing heart conditions after an mRNA Covid vaccine is very low, and should not call into question the value of life-saving vaccination campaigns rolled out across the world.
But the risk is “non-negligible”, and the available evidence is difficult to assess precisely, researchers have said, calling for better data on the subject.
“That we are now more than a year and a half into mass mRNA vaccination and still do not have strong certainty about the incidence of this clinically important outcome is disappointing,” US researchers Jing Luo and Walid Gellad wrote in a commentary in the BMJ journal in mid-July.
Their opinion piece was based on a large study in the same issue which reviewed the available research about the frequency of myocarditis and pericarditis, two inflammations in the heart, following vaccination with Pfizer and Moderna’s mRNA shots.
It has been a delicate subject for the scientific community. The risks were quickly identified after the launch of mass vaccinations campaigns last year, but were then greatly exaggerated by vaccine sceptics seeking to oppose all jabs.
The heart conditions have been very rare after vaccination and most of the time have not caused serious complications. The fact that Covid itself presents cardiovascular risks is also part of the equation.
– ‘Question of major importance’ –
So the value of mRNA vaccines has not been called into question. But for the researchers writing in the BMJ, the current state of knowledge about the risks remains inadequate.
“Clearly, the incidence of myocarditis is rare after vaccination,” they wrote. “Just how rare remains a question of major importance.”
The review they commented on, in which other researchers analysed the results from around 50 previous studies, provides some answers.
It largely confirmed previous findings, including that the risk of myocarditis after vaccination appears to be higher for young men than any other group.
And, though the conclusions are less clear, there also seems to be a slightly higher risk after a dose of Moderna’s vaccine, compared to Pfizer’s.
That would appear to bolster the decision by some countries, including France, to only allow people over 30 to get the Moderna Covid vaccine. Other countries such as the United States have made no such distinction.
The research pointed towards a lack of clarity in other areas.
It found that some results could vary widely depending on different methodologies while others could be seen as unsatisfactory.
That was the case for a particularly sensitive topic: vaccinating children.
Numerous countries have authorised vaccination for young children, but uptake has often been sluggish due in part to the reluctance of parents.
Myocarditis in children aged five to 11 years is “very rare”, the researchers said, but added that the “certainty was low”.
– Boosters less of risk? –
The researchers raised other issues.
Most cases of myocarditis and pericarditis were resolved without serious illness. But they were rarely followed over a long period of time, meaning they could not rule out future complications.
The research was also unclear about whether a booster dose posed as much risk as initial vaccination.
It could be a pressing question as many developed countries with largely vaccinated populations consider how often they should roll out additional doses.
However a French study released on Friday shed some light on the subject.
The study, which has not yet been peer-reviewed or published in a scientific journal, was conducted by Epi-Phare, part of France’s medicine safety agency ANSM, looking at the public health data of millions of French people.
It found less risk of myocarditis from booster doses of Pfizer and Moderna Covid vaccines than during initial vaccination.
“In addition, the risk decreases with the length of time between each successive dose,” the researchers said.