Myocarditis By the Numbers – Part 3 
Health Matters, May 2024

Myocarditis By the Numbers – Part 3 

By Caitlin Rothermel

New information is now coming out quickly regarding myocarditis caused by the mRNA vaccines. On April 16, 2024, the United States National Academies of Sciences, Engineering, and Medicine (NASEM – formerly the National Academy of Science) released an expert report finding a causal relationship between the SARS-CoV2 mRNA vaccines and myocarditis. 

The language is meaningful. In previous myocarditis articles, I covered research showing that the injections were associated with myocarditis. Now, the NASEM has looked at the evidence and determined it is reasonable to say the mRNA vaccines caused myocarditis. 

What makes for causality? For the NASEM experts, it required a “yes” to three key questions:

  • Have we seen myocarditis occurring in the time frame after vaccination?
  • Has a mechanism of action for how the vaccine could cause this damage been proposed, and does it look feasible? 
  • Has the injury (spike protein from the injection) been found in the heart tissue of patients?

This is a pretty definitive finding, achieved after careful evaluation. But many other causation stories still need to come clear for the mRNA vaccines. For example, I keep hearing and reading that more myocarditis cases happen after COVID disease than after mRNA injection. 

That’s a pretty bold statement of causation, and yet it is said so casually. I decided to do some research. First of all, I learned that many articles that make this statement never indicate where their evidence is from, something that would typically be considered deceptive. Secondly, when evidence is cited, there are two studies most often mentioned; it’s worth knowing what they say:

The first is a self-controlled case series from the United Kingdom, conducted from December 2020 to August 2021. The investigators combined data from the English National Immunization database with patient information to look at hospitalizations or deaths from myocarditis, pericarditis, and cardiac arrhythmias. 

You’ll want to pay attention to how the cause of myocarditis was defined in this study. Myocarditis was considered injection-related if the patient had received a vaccine in the past 28 days, and myocarditis was considered COVID disease-related if the patient had a positive COVID test in the past 28 days and had not received an injection in that time period. After the second injection and after COVID infection, respectively, investigators found an increase of 1 and 4 events per 100,000. 

There are many reasons why this study does not make sense as definitive evidence. Self-controlled case series are usually used to evaluate vaccine safety early on after a product is released. But since it has been a few years since the original injections were administered, a more current comparative analysis, looking at a large number of U.S. residents, would be more definitive.

From this perspective, the second study – a U.S. cohort analysis, conducted by the National Patient-Centered Clinical Research Network – makes more sense. It used electronic health records from 40 healthcare systems to track more than 7 million patients from January 2021 to January 2022. Similar to the UK analysis, the myocarditis was defined as injection-related if an mRNA vaccination was given in the past 30 days, and was considered COVID-related if there was a positive COVID test in the past 30 days and no mRNA injection in that time. 

This study found dramatically higher rates of myocarditis with COVID infection than following a second injection – at least a doubling. But these numbers are also dramatically overblown because this database didn’t capture the patients who caught COVID and tested at home, or at a pharmacy, or who didn’t test at all. These unrepresented people were at lower risk in general, and because of this, the study actually shows results for higher-risk COVID patients – people were more likely to have side effects like myocarditis. 

There is another problem with these studies, related to causality, and it needs to be considered. This research happened after December 2020, when the mRNA injections were authorized for release. Due to this, many if not most study participants were vaccinated while the research took place. And while these studies both assumed that previous mRNA vaccination did not contribute to myocarditis beyond the 28 or 30 days that followed, that’s just an assumption; there’s no biologic explanation used to support it.

By overlooking this, researchers introduced what’s known as confounding – a type of bias that happens when factors that could affect study results are ignored. People who are vaccinated and then have a COVID infection almost certainly have a different biologic (and risk) profile than people with only an infection or vaccination – just like patients with two injections have different risk profiles from patients with one or three. In all these scenarios, the body is exposed (and re-exposed) to spike protein. Many people fall into the category of vaccination followed by infection, because having the mRNA vaccination only provides a few months of protection from COVID. 

I don’t think the evidence does a good enough job supporting the contention that myocarditis happens more often after COVID disease than mRNA injection. But at this point, it’s probably a fool’s errand to study COVID infection and vaccination as separate risk factors, since so many people have experienced both. To help us understand how our bodies are fundamentally changed in the “new normal,” research is going to have to meet us where we are at. 

May 9, 2024

About Author

caitlin I’m a member of the Vashon Loop Editorial Board and write about medicine, health, and society. I’m a research geek and an MPH, and I’m also a mom, farmer, teacher, and apocalypse librarian. I edit things. If I’m not doing something, it’s probably because I am asleep.