Welcome to Impact Factor, your weekly dose of commentary on a new medical study. I'm Dr F. Perry Wilson of the Yale School of Medicine.
Let me start with a noncontroversial statement: A small number of individuals who receive an mRNA vaccine will develop myocarditis. Myocarditis is a side effect of these vaccines, albeit a rare one.
It is a side effect that has taken on an importance in the public consciousness that far outpaces its actual rate. For many people, it serves as confirmation that their worst fears about the vaccines — that they were rushed through development without proper oversight, that they are part of a plot to depopulate the planet, that they contain tracking devices — are true.
And we could try to explain how rare outcomes are rare, or how the risk for myocarditis from COVID is likely higher than the risk for myocarditis from vaccines, or how myocarditis is not the only outcome we need to worry about when it comes to COVID infection. But in the end, the fact that some people will experience myocarditis after vaccination gives an off-ramp to the vaccine hesitant, hindering public health efforts.
But according to a paper appearing in the Journal of the American College of Cardiology, not all vaccines are created equal.
This paper compares myocarditis rates between the Pfizer and Moderna vaccines. And the differences are pretty stark.
This is a huge study of more than 3 million individuals who received at least two doses of mRNA vaccine in British Columbia in 2021. Just over 70% received the Pfizer vaccine.
Researchers linked those vaccine records to emergency room and hospitalization visits for myocarditis and pericarditis (as defined by ICD-10 codes) within 21 days of vaccination. An aside here: Yes, ICD-10 codes are imperfect, but it seems like they were specific in this study. Of the 59 cases of myocarditis identified, 57 had a troponin measured and 51 of those were elevated. Of course, it's possible that there are more cases of myocarditis that were never captured, either because the individual did not seek care or the diagnosis was missed.
From here, the study is simple. The numerator is the number of cases of myocarditis; the denominator is the number of people vaccinated. And here's what you get.
Lots to take home from this graph. First of all, let's get oriented to the Y-axis. The background rate of myocarditis, pre-pandemic, pre-vaccine, is about 2 per million. So, clearly, the signal you see here is not just random background noise. As in prior studies, you can see that the risk is highest among men, and particularly among younger men. But, critically, you can see that the rate of myocarditis in those at-risk groups is dramatically higher with the Moderna vaccine compared with Pfizer.
That point is driven home in this time-to-event graph.
You can see that myocarditis cases start to rise within a couple of days of vaccination with Moderna, plateauing after a week or so. Pfizer, while clearly above baseline, has a substantially lower rate.
If we focus in on those young men, you can see how the rates in this study compare with those measured by the CDC, and, for good measure, the rate seen in young college athletes recovering from a COVID infection.
Again, it does seem that the risk from COVID far outpaces the risk from vaccination, but that risk outpaces the background rate of myocarditis.
What is a young man to do? To be honest, it's complicated and this study does not answer all the questions that we may have about this situation. There was no real control group; it did not measure the rate of myocarditis among people who don't get the vaccine, though certainly the pre-COVID rate is quite a bit lower. There was no analysis of post-COVID myocarditis; I'm pulling numbers from other studies to make a comparison there. This study didn't look at other side effects from vaccination or the efficacy of vaccination against COVID infection, hospitalization, or death. It didn't look at the myocarditis rate among those with a prior COVID infection, or after a booster vaccine. Finally, though other studies have suggested that postvaccine myocarditis is relatively mild, this one doesn't tell us how these patients fared.
But it did tell us something really important: that there is a difference in the rate of this particular adverse event based on which mRNA vaccine is received. And for those who are going to get vaccinated, that may be a very useful piece of information. Particularly for young men, it seems like Pfizer is a better choice here. It may well be the lesser of three evils.
F. Perry Wilson, MD, MSCE, is an associate professor of medicine and director of Yale's Clinical and Translational Research Accelerator. His science communication work can be found in the Huffington Post, on NPR, and here on Medscape. He tweets @fperrywilson and hosts a repository of his communication work at www.methodsman.com.
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Cite this: F. Perry Wilson. Which mRNA Vaccine Poses a Lower Risk for Myocarditis? - Medscape - Nov 08, 2022.