The risk of rare heart inflammation following Covid-19 vaccination is “very low,” and much smaller than the risks linked to a coronavirus infection itself, according to a peer reviewed study published on Monday in Lancet Respiratory Medicine, underscoring the benefits and safety profile of Covid vaccines as many countries begin to roll out second booster doses to protect against new waves of the virus.
The overall risk of myopericarditis—the inflammation of the heart muscle and lining around the heart—following Covid vaccination is low and not significantly different from people receiving vaccines against other diseases, according to the analysis of 22 studies using international databases from 1947 through December 2021.
The analysis, which covered around 395 million doses of Covid-19 vaccine, including around 300 million mRNA vaccines by Pfizer and Moderna and nearly 10 million doses of non-Covid vaccines, found 18 cases of myopericarditis per million doses of Covid-19 vaccine compared to 56 cases per million doses for non-Covid vaccinations like influenza.
The risk of myopericarditis was nearly four times higher among those receiving an mRNA Covid vaccine versus a non-mRNA Covid vaccine and among those getting their second dose compared to a first or third, the researchers found.
The risk was notably higher among young men under 30, who had rates 10 times higher than women of the same age, the researchers said, with the rate for men over 30 around three times as high than for women over 30.
Study co-author Dr. Jyoti Somani, an infectious diseases specialist at National University Hospital, Singapore, said the findings should highlight the fact that the risks of rare adverse events “should be offset by the benefits of vaccination, which include a lower risk of infection, hospitalization, severe disease, and death.”
Covid-19 carries a more significant risk of heart issues alongside a litany of other problems. The researchers found 1.1% of 2.5 million people hospitalized with Covid-19 showed signs of myopericarditis. While not directly comparable to the vaccine findings—the issues were measured differently—the rate points to a much higher incidence among those infected with the virus than those receiving the vaccine. The finding is in line with other studies on the issue, including by the Centers for Disease Control and Prevention.
What We Don’t Know
Why heart issues are linked to mRNA Covid-19 vaccines. Heart inflammation, especially among young men, was an unexpected adverse event linked to mRNA shots. Its cause is not known and could be a “side effect of the inflammatory processes induced by any vaccination” said Somani. It’s not necessarily unique to the virus that causes Covid-19, Somani added. While unexpected, the issue is not unprecedented, noted Dr. Margaret Ryan of the Defense Health Agency in a linked commentary, pointing to high rates of myopericarditis among young men after smallpox vaccination. Ryan, who is also a clinical professor at the University of California San Diego, said the finding with mRNA vaccines should be used to “propel the field forward” and “spur the development of safer vaccines or precision vaccination practices.”
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