This article was originally published in the Montreal Gazette
The hard part about vaccines is that you have to convince people to get them when they are healthy. Reports of side effects shake people’s confidence. That’s why many parents are worried about vaccinating their children after hearing about a link between the Pfizer vaccine and myocarditis in young men. What most news reports failed to mention, though, is how rare and mild these cases actually were.
Myocarditis literally means inflammation of the heart muscle. It is not a common condition, nor is it rare. In any given year, there are four to six cases per 100,000 people. It can be caused by many different things, but it is often due to a viral infection that causes swelling and inflammation of the heart. Most cases resolve and heart function often returns to normal. A small number of people are left with long-term cardiac dysfunction.
Recent reports have suggested that there may be a wave of cases in young men and boys following their second vaccine dose. While data on this issue is scarce, we do have some numbers that we can work from. Israel’s Ministry of Health released surveillance data on cases of myocarditis between December 2020 and May 2021. They determined that there were 275 cases of myocarditis in the country over that time period. But, importantly, only 148 of these cases occurred around the time of vaccination, with most occurring after the second dose. Hospitalizations were generally short, under four days, and 95 per cent of cases were considered mild.
What was often missing from news reports is that the 148 cases in question occurred in the context of more than five million people being fully vaccinated, which amounts to an incidence of between two and three thousandths of a per cent. Cases of myocarditis post-vaccination are exceedingly rare according to this Israeli data. While the ministry said there was a “possible link,” it acknowledged that even if true, most cases were mild illnesses that passed within a few days.
U.S. data on the issue is more sparse. The Centers for Disease Control has said that there have been reports of myocarditis in the United States after people received an mRNA vaccine like Pfizer’s or Moderna’s, but they have not released detailed numbers. Interestingly, the analysis of different vaccine databases has yielded conflicting conclusions. Analysis of the Vaccine Adverse Event Report System (VAERS) apparently did show more cases of myocarditis especially among young men ages 16 to 24, while data from the Vaccine Safety Datalink (VSD) did not.
There is, however, an important difference between these two data sources. VAERS is a reporting system, meaning that anyone can report a vaccine side effect. These reports are not verified and can come from anyone, not just the patient. The VSD, by contrast, uses electronic health data compiled from nine health care systems across the United States, and therefore myocarditis cases in the VSD were probably confirmed in hospital. The fact that the link between the vaccines and myocarditis was seen in VAERS but not VSD would suggest that the link may not be real.
The only U.S. numbers we can draw on is a recent case series in Pediatrics that described seven cases of myocarditis following vaccination in boys ages 14 to 19. Given that more than five million COVID-19 vaccines have been administered in this age group in the United States, that would represent a trivially small risk, though the authors could not be certain they had identified every case in the country.
What remains unclear, despite all this data, is whether these cases of myocarditis were actually caused by the vaccine or whether they represent the normal background rate of the disease that would have happened anyway. If the risk is real, it appears to be extremely low. In contrast with COVID-19, where the risk is definitely real and anything but negligible.