In practice

• As of late October 2021: myocarditis is very rare, but must be taken into account, especially in young men. In the published results of the comparative trials conducted to obtain marketing authorisation for the covid-19 messenger RNA vaccines, which included tens of thousands of persons, myocarditis was rare in the vaccinated groups and in the control groups, and the trials lacked the statistical power to demonstrate such a rare adverse effect.
   
• A very weak safety signal for myocarditis linked to vaccination with covid-19 messenger RNA vaccines was detected once hundreds of millions of people had been vaccinated, and adverse events had been monitored and reported to pharmacovigilance systems by healthcare professionals and patients. Studies based on health databases then confirmed and quantified this safety signal.
   
• This adverse effect is less rare in men than in women, in the young than in older vaccinees, after the second dose than after the first dose, and possibly with elasomeran (containing 100 micrograms of messenger RNA) than with tozinameran (containing 30 micrograms of messenger RNA). It is difficult to be more specific about these risk factors as of late October 2021 due to the effect's rarity. It is not yet known whether or not the incidence is higher after a third dose, given several months after the first two injections. Nor is it known whether the risk will differ with the half-dose of elasomeran (50 micrograms of messenger RNA) that was authorised in the European Union in late October 2021. And it is not known whether myocarditis induced by these vaccines leaves any long-term sequelae.
   
• The symptoms suggestive of (although not specific for) myocarditis during the days that follow administration of a covid-19 messenger RNA vaccine are mainly: fatigue, chest pain, shortness of breath, signs of heart failure or even cardiogenic shock, and arrhythmia.
  
  
• This adverse effect can be very bothersome and sometimes serious, but it appears extremely rare for a patient to die from myocarditis linked to these vaccines. In contrast, as of early November 2021, covid-19 messenger RNA vaccines remain an option with proven efficacy for reducing the risk of developing covid-19, especially severe disease, provided there is no local or regional emergence of variants resistant to these vaccines.
   
• In the context of the 2020-2021 epidemic, the mortality associated with vaccine-induced myocarditis appears lower than that associated with covid-19, even for young men. Covid-19 vaccines are an addition to, but not a replacement for, other preventive measures.

©Prescrire 1 February 2022

Source: "Update on myocarditis linked to the covid-19 messenger RNA vaccines tozinameran (Comirnaty°) and elasomeran (Spikevax°)" Prescrire International 2022; 31 (234): 44-45. Subscribers only.

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