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The estimated incidence of myocarditis in patients who received at least one dose of Pfizer-BioNTech mRNA vaccine (BNT162b2) against coronavirus disease 2019 (COVID-19) was 2.13 cases per 100,000 people, affecting mainly young men and presenting mainly as mild or moderate illness, as demonstrated by a recent study published in the New England Journal of Medicine.
In the ongoing battle against the COVID-19 pandemic, Israel has conducted one of the fastest vaccination campaigns to date. Specifically, as of October 2021, over 60% of the population was fully vaccinated. Yet, not much was known about the rare but possible side effects of the vaccines currently in use.
On May 27, 2021, there was a statement from the United States Centers for Disease Control and Prevention (CDC) on a possible link between vaccination and myocarditis (meaning inflammation of the heart muscle) – both for the Pfizer-BioNTech BNT162b2 vaccine and Moderna mRNA-1273 vaccine.
Six days later, the Israeli Ministry of Health revealed that 148 cases of myocarditis had been recognized in Israel between December 2020 and May 2021, which coincided with the start of the vaccination program. This was particularly relevant for the link between the second dose of the vaccine and myocarditis in male patients aged 16 to 30 years.
This is why an Israeli research group, led by Dr Guy Witberg of Beilinson Hospital in Petah Tikva and Tel Aviv University, set out to assess both the burden and severity of myocarditis after vaccination. in order to obtain more data on this question.
Identify cases of myocarditis in vaccinees
This retrospective cohort study was based on data from the only and largest healthcare facility in Israel – Clalit Health Services. Covering a total of 4.7 million patients (representing 52% of the population), the aforementioned organization provides full outpatient care, while inpatient care is provided by network and non-network hospitals.
The study population included people registered with Clalit health services and vaccinated between December 20, 2020 and May 24, 2021. Researchers identified suspected cases of myocarditis within 42 days of the first dose of the vaccine. Clinical course, presentation and results were extracted from electronic patient health records.
The diagnosis of myocarditis was made using the case definition according to the CDC. In addition, myocarditis was classified as fulminant or non-fulminant, and the latter were further classified as mild or intermediate, taking into account all currently valid definitions.
Young men most at risk
In short, among more than 2.5 million vaccinated people over 16 years of age, there were a total of 54 cases that met all the criteria necessary for a diagnosis of myocarditis. This resulted in an estimated incidence of 2.13 cases per 100,000 people who received at least one dose of Pfizer-BioNTech mRNA vaccine (BNT162b2).
The highest incidence of myocarditis, 10.69 cases per 100,000 people, was reported in male patients aged 16 to 29 years. Additionally, a total of 76% of myocarditis cases were mild and 22% intermediate, while there was one case related to a life-threatening condition known as cardiogenic shock.
After a median follow-up period of 83 days, one patient had been readmitted to hospital with myocarditis and another had died of unknown cause. In addition, out of 14 patients with left ventricular dysfunction upon admission, ten of them still had such dysfunction upon discharge from hospital (and five of them underwent subsequent tests which resulted in revealed normal heart function).
Implications and need for more studies
“Although we cannot directly compare the incidence of post-vaccination myocarditis in our study with the incidence in other studies, our data may provide benchmarks,” explain the authors of this article. New England Journal of Medicine paper.
Nevertheless, the follow-up period in this cohort of patients was relatively short to determine the long-term prognosis of patients with myocarditis after vaccination, and there are several other limitations of the study (such as the lack of a group of simultaneous comparison, lack of validation of myocarditis with biopsies, or lack of specific details in hospital or discharge notes).
In any case, this study reinforces the need for continued active surveillance of myocarditis cases after vaccination, as there may be a significant association between mRNA vaccines and myocarditis in younger individuals.
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