Mortality Higher in Myocarditis After Virus Than After COVID Vax

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Myocarditis cases following administration of the Pfizer-BioNTech mRNA COVID-19 vaccine have a significantly lower rate of mortality compared with myocarditis associated with viral infection, a new registry study suggests.

Investigators examined data from Hong Kong Territory national health registry to compare 6-month outcomes from patients who developed myocarditis following administration of the Pfizer-BioNTech COVID-19 vaccine (BNT162b2) with those in a historical control group of non–COVID-19 viral myocarditis cases that occurred prior to the pandemic.

They identified 104 patients with myocarditis following COVID-19 vaccination, and 762 historical patients with non–COVID-19 viral infection–related myocarditis.

Results showed that there was one death (1%) in the 104 patients with post-vaccination myocarditis compared with 84 deaths (11%) in the 762 patients with viral infection–related myocarditis.

Adjusted analysis showed that the postvaccination myocarditis group had a 92% lower mortality risk compared with the historical controls (adjusted HR, 0.08; 95% CI, 0.01 – 0.57).

One case (1%) of dilated cardiomyopathy and 2 cases (1.9%) of heart failure were identified in the postvaccination group, compared with 28 (3.7%) and 93 (12.2%) in the viral infection–related myocarditis group, respectively.

No significant differences in other prognostic outcomes were seen.

The study is published in the December 13 issue of the Journal of the American College of Cardiology, and was published online today.

“We observed very low incidence rates (less than 1 per 10,000 person-days) of mortality, heart failure, and dilated cardiomyopathy following myocarditis after mRNA vaccination, in contrast with incidence rates of 7, 8, and 2 per 10,000 person-days, respectively, among the viral infection-related myocarditis patients,” the authors, led by Francisco Tsz Tsun Lai, PhD, The University of Hong Kong, say.  

They note that almost 4 million individuals have received the Pfizer-BioNTech COVID-19 vaccine in Hong Kong. The 104 myocarditis cases identified in this study as possibly associated with the vaccine works out to a rate of 2.6 cases per 100,000 persons, which they point out is comparable to previous estimates in other populations.

The authors also report that the study’s findings are consistent with the existing literature in other respects. First, the demographic features of the cohort of patients with myocarditis following mRNA vaccination were characterized by a higher proportion of males and were generally younger; and second, the prognosis was typically mild among those with myocarditis following mRNA vaccination, with very few deaths or other adverse prognostic outcomes recorded.

“Nevertheless, this study is the first, to the best of our knowledge, comparing the prognostic outcomes of myocarditis related to mRNA vaccines versus myocarditis related to the viral infection,” they say.

“Importantly, a significant and substantially lower rate of mortality was found among the patients with myocarditis following mRNA vaccination compared with those with viral infection–related myocarditis.”

They suggest that because this difference has already been adjusted for a variety of clinical histories, medication use, and demographic information, this finding may suggest a potentially distinct etiology of myocarditis conditions related to mRNA vaccines as distinguished from those otherwise acquired such as viral infection.

“Considering immunology, it may be sensible to anticipate a milder prognosis because of typically brief exposure to stimuli that triggered the immune responses (ie, vaccine vs viruses),” they add.  

They also point out that the underlying health conditions, particularly cardiovascular health status, of the patients with myocarditis following mRNA vaccination were better than the patients with viral infection–related myocarditis, across the age groups.

“This finding potentially implies that the incidence of myocarditis related to viral infections is typically higher among those with an underlying medical condition,” they comment. “It therefore also reflects the iatrogenicity of the myocarditis cases that were related to mRNA vaccines, which could apparently occur in otherwise healthy individuals. The low incidence of mortality, heart failure, and cardiomyopathy may be partly explained by this observation.”

“Reassuring Initial Look”

In an accompanying editorial, Peter P. Liu, MD, and Tahir S. Kafil, MD, University of Ottawa, Ottawa, Canada, point out some limitations of the current study.

These include the absence of standardized case-definition criteria for vaccine-associated myocarditis and the investigators were not able to confirm the myocarditis diagnoses with clinical investigative data such as cardiac MRI; nor were they able to confirm exclusion of other potential causes of myocarditis.

They also point out that the comparator group was “not ideal” as viral myocarditis is a very heterogeneous group of conditions influenced by local seasonal viral patterns and underlying population comorbidities.

They suggest that a better comparator would be COVID-19–induced myocarditis, and they report that the US National Patient-Centered Clinical Research Network (PCORnet) has examined records of more than 15 million patients from 40 healthcare systems and found that the risk for adverse cardiac events such as myocarditis/pericarditis in young males is still 1.8 to 5.6 times higher after COVID-19 infection than from COVID-19 vaccination.

Noting that the mechanism behind this vaccine-associated myocarditis is not known, the editorialists say that, given the anticipated need for regular COVID-19 booster vaccinations and advancements in mRNA technologies for various other medical indications, vaccine myocarditis will continue to be an ongoing challenge into the foreseeable future.

They point out the need to work collaboratively to capture the longer-term outcomes in these patients, to identify the specific individual risk factors leading to the development of myocarditis, and mitigation strategies for those who are affected.

“Joining global collaborations will be critical for our collective success. Whereas this present study provides a reassuring initial look at 6-month outcomes data after BNT162b2 vaccination, it is not yet time to roll down our sleeves,” they conclude.

This study was funded by a research grant from the Food and Health Bureau, the Government of the Hong Kong Special Administrative Region. Lai reports research funding from the Laboratory of Data Discovery for Health, funded by [email protected] administered by the Innovation and Technology Commission, the RGC Postdoctoral Fellowship under the Hong Kong Research Grants Council; and the Food and Health Bureau of the Government of the Hong Kong Special Administrative Region.

J Am Coll Cardiol. Published online December 5, 2002. Abstract, Editorial

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