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Fact Check-What we know about myocarditis, COVID-19 infection, and COVID-19 vaccines so far

Corrects typo in paragraph 25

Recent research about myocarditis, a severe inflammation in the heart that has been linked to both COVID-19 infections and rarely to COVID-19 vaccination with mRNA vaccines, has led to heated debate online.

Evidence so far on myocarditis associated with COVID-19 vaccination suggests that the incidence of cases remains low overall. Moreover, when it does occur, primarily in young males, studies show cases are usually mild and resolved quickly.

Experts continue to assure the overall risks of COVID-19 infection far outweigh the risks associated to developing myocarditis after a COVID-19 vaccination, even in young men.

WHAT IS MYOCARDITIS

Myocarditis is a rare inflammation of the heart muscle, the myocardium ( here ), ( here ). It is a result of the body’s immune response to a trigger, such as an infection.

The inflammation is most commonly caused by infections —mainly viral, including the viruses that cause the flu, the common cold and COVID-19, bacteria or fungus— but can also be triggered by certain medicines, autoimmune diseases or environmental conditions. In rare cases, myocarditis complications may cause an irregular heartbeat (arrhythmia) or heart failure, the U.S. National Heart, Lung and Blood Institute explains ( here ).

IS MYOCARDITIS A COVID-19 VACCINE SIDE EFFECT?

Cases of myocarditis following mRNA COVID-19 vaccines have been reported in the United States ( here ) and other countries ( here ).

The U.S. Centers for Disease Control and Prevention (CDC) lists myocarditis and pericarditis (inflammation of the outer lining of the heart) as a “serious type of adverse event after COVID-19 vaccination.” ( here , here ) Most of the cases have been reported after receiving a Pfizer-BioNTech or Moderna vaccine.

“Any health problem that happens after vaccination is considered an adverse event. An adverse event can be caused by the vaccine or can be caused by a coincidental event not related to the vaccine,” the CDC states ( here ).

WHO IS AT RISK FROM MYOCARDITIS?

Myocarditis can affect anyone across age groups, but it occurs more often among young males, from puberty through their 30’s, the Myocarditis Foundation, a U.S. non-profit organization, explains ( here ). Meanwhile, cases of myocarditis triggered by autoimmune diseases, like lupus, are more common in women, according to the U.S. National Heart, Lung and Blood Institute ( here ).

So far, cases of myocarditis reported after COVID-19 vaccination have followed this same trend in age and sex.

Two large studies in Israel published on the New England Journal of Medicine on Dec. 2, 2021 ( here ), ( here ), observed a higher incidence of cases in young males who received mRNA vaccines.

An analysis of VAERS reports in June by the CDC Advisory Committee on Immunization Practices (ACIP) (see page 17, here) similarly identified a predominance of reported cases in male patients in younger age groups, often after a second dose of an mRNA COVID-19 vaccine (see page 27, 18- to 24-year-olds).

Another analysis of reported cases among 5- to 15-year-olds found the majority of incidences were concentrated in males ( here ). In an updated analysis of VAERS reports published in JAMA this week, CDC researchers found a small but increased risk of myocarditis following shots of mRNA COVID-19 vaccines from Pfizer-BioNTech and Moderna in multiple age groups but largely in adolescent and young adult males. ( here )

For every million doses of Pfizer’s vaccine, there were roughly 71 cases of myocarditis in males aged 12 to 15 and 106 cases in males aged 16 or 17. In young men aged 18 to 24, the rates per million doses were roughly 52 with the Pfizer shots and 56 after Moderna shots.

The latest CDC guidance states that cases have especially occurred in male adolescents and young adults, more commonly following the second dose of the vaccine, usually within a week of vaccination ( here ).

COMPARING MYOCARDITIS RISK

Some studies have looked at the risk of myocarditis following vaccination in comparison to the risk of myocarditis following infection.

A British study published on Dec. 14, 2021 found that COVID-19 infections are more likely to trigger rare cardiovascular complications, such as myocarditis, pericarditis and cardiac arrhythmia, than vaccines ( here ).

The study led by Oxford University researchers and published in Nature Medicine found an increased risk of myocarditis with first doses of AstraZeneca-Oxford and Pfizer-BioNTech vaccines and both doses of Moderna's shot, but the risk was much higher after a COVID-19 infection ( here ).

A subsequent pre-print study by the same team of researchers, published on Medrxiv on Dec. 25, 2021 ( here ), looked into cases by age and sex.

The analysis, yet to be peer-reviewed, found a “notable exception” in men younger than 40 following a second dose of the Moderna vaccine. For this subgroup, the risk of myocarditis was “higher than following a COVID-19 infection.” Researchers estimated an additional 101 events of myocarditis per million people vaccinated with the second dose, compared with seven events per million following a positive SARS-CoV-2 infection.

Both the pre-print and the Nature Medicine study, however “found that the risk of hospitalization and death from myocarditis following COVID-19 infection is higher than the risk associated with vaccination in the overall population,” Martina Patone, Data Scientist at the Nuffield Department of Primary Health Care Sciences at the University of Oxford and one of the authors of the studies told Reuters via email.

Contacted by Reuters, James A. de Lemos, M.D. and co-chair of the American Heart Association’s COVID-19 CVD Registry ( here ), weighed in saying the pre-print study from the UK highlights that “while the relative risk of myocarditis increases after vaccination, the absolute risk remains extremely low, even among young males.”

“The focus on comparative risks of myocarditis after vaccination and COVID-19, without considering the full spectrum of illness from COVID-19, is misguided and disregards the importance of balancing benefits and risks of vaccination,” Lemos said.

Another Israeli study published on Sept. 16, which evaluated the safety of the Pfizer-BioNTech COVID-19 vaccine and its potential adverse events ( here ), observed an increased incidence of myocarditis following vaccination, but also concluded that “SARS-CoV-2 infection is itself a very strong risk factor for myocarditis, and it also substantially increases the risk of multiple other serious adverse events.”

Mariell Jessup, the American Heart Association’s Chief Science and Medical Officer, further stressed that the benefits of COVID-19 immunization “in preventing the full spectrum of short-and long-term complications of COVID far outweigh the small risk of myocarditis after vaccination, even in young males.”

For example, children aged 5-11 years old are at highest risk for MISC-C, a rare but often serious condition in children that causes organ inflammation weeks after a mild or asymptomatic infection ( here ) that has been linked to COVID-19. According to the American College of Cardiology, about 65% of patients with MISC-C require critical care and medications to support blood pressure ( here ), ( here ).

A study in the U.S. published on Dec. 6, 2021 ( here ) that evaluated 139 adolescents and young adults with suspected myocarditis within 30 days of COVID-19 vaccination also found cases are usually mild and have a “rapid resolution of symptoms.”

This is consistent with another analysis by the CDC ( here ), which evaluated 265 verified reports in VAERS of myocarditis among 12- to 15-year-olds after COVID-19 vaccination. About 92% of the reported cases among this age group had recovered at the time of report.

European and U.S. regulators and the World Health Organization have said that the benefits of mRNA shots in preventing COVID-19 vaccines continue to outweigh the risks ( here ).

The U.S. Centers for Disease Control and Prevention continues to recommend COVID-19 vaccination to everyone aged 5 years and older ( here ).

This article was produced by the Reuters Fact Check team. Read more about our fact-checking work here.

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