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The Pfizer COVID-19 mRNA vaccine was found to be associated with a threefold increased risk of myocarditis, according to a real-world case-control study from Israel.

Vaccination had a strong association with an increased risk of myocarditis (risk ratio [RR] 3.24, 95% CI 1.55-12.44, risk difference 2.7 events per 100,000 persons), as well as increased risks of lymphadenopathy (RR 2.43, 95% CI 2.05-2.78, 78.4 per 100,000), appendicitis (RR 1.40, 95% CI 1.02-2.01, 5.0 per 100,000), and herpes zoster infection (RR 1.43, 95% CI 1.20-1.73, 15.8 per 100,000), reported Ran Balicer, MD, of Clalit Health Services in Tel Aviv, and colleagues.

However, in a separate cohort, infection with SARS-CoV-2 was associated with a higher risk of myocarditis (RR 18.28, 95% CI 3.95-25.12, 11.0 per 100,000), as well as other cardiovascular complications, including acute kidney injury (RR 14.83, 95% CI 9.24-28.75, 125.4 per 100,000), pulmonary embolism (RR 12.14, 95% CI 6.89-29.20, 61.7 per 100,000), and intracranial hemorrhage (RR 6.89, 95% CI 1.90-19.16, 7.6 per 100,000), the authors wrote in the

They noted that vaccination was "substantially protective" against anemia, acute kidney injury, intracranial hemorrhage, and lymphopenia.

Balicer's group examined data from the largest healthcare organization in Israel to compare incidence of adverse events among vaccinated individuals versus unvaccinated individuals, and estimated the effects of SARS-CoV-2 infection on these adverse events.

Participants in the vaccination cohorts were 16 years old and older, had been in the health organization for a full year, had no prior COVID-19 infection, and had no contact with the healthcare system in the last 7 days. Notably, populations with confounders, such as healthcare workers, long-term care facility residents, or people confined to their home for medical reasons, were excluded.

While it may be tempting to compare the risk differences between vaccination and infection, the authors cautioned against this.

"The effects of vaccination and of SARS-CoV-2 infection were estimated with different cohorts," they wrote. "Thus, they should be treated as separate sets of results rather than directly compared."

From Dec. 20, 2020 to May 24, 2021, eligible people vaccinated on a particular day were matched to eligible unvaccinated controls by age, sex, place of residence, socioeconomic status, and population sector. The study included 21 days of follow-up after the first and second doses of Pfizer vaccine. For each adverse event, patients were followed from the day of matching until documentation of the adverse event, 42 days, the end of the study period, or death.

To "place the magnitude of the adverse effects of the vaccine in context," Balicer and team also estimated the effects of SARS-CoV-2 infection on these same adverse effects during the 42 days after diagnosis.

Overall, 884,828 people each were included in the vaccination cohort and the unvaccinated cohort, though 235,541 in the unvaccinated cohort had to be rematched following vaccination. The researchers also included 173,106 people with COVID-19 infection matched with the same number of uninfected people.

The median age of the eligible cohort of 1,736,832 people was 43, and 48% were women. Median age in the vaccination cohorts was 38. Median age of the infection cohort was 36, and 54% were women.

Limitations to the study included that study participants were not randomly assigned according to exposures, which could introduce confounding and bias, and that the matching process resulted in a study population whose median age was 5 years younger than the eligible population. In addition, certain high-risk populations were excluded from the study.

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