How big is the myocarditis risk by age and dose number?

The short version: myocarditis risk after mRNA COVID-19 vaccines is concentrated in young males, especially after the second dose, with Moderna showing higher rates than Pfizer, and booster doses carrying lower risk than the primary series.

The risk is not uniform across age or dose. For males aged 12-17, a meta-analysis of four studies found about 16 cases per 100,000 after the second Pfizer dose, compared to about 2 per 100,000 after the first [5]. For females the same age, the second-dose rate was about 6 per 100,000 [5]. A living BMJ systematic review reported a wider range for males 12-17: 50 to 139 cases per million (5 to 14 per 100,000), with moderate certainty that Moderna carries a higher risk than Pfizer in 18-29 year olds [6].

The second dose is the highest-risk point. In England, the relative incidence of myocarditis admission in 16-39 year olds was 5.3 times baseline after a second Pfizer dose and 56 times baseline after a second Moderna dose [3]. Booster doses showed lower relative incidence: about 3 times baseline for Pfizer and 6 times for Moderna [3]. A US VAERS study found bivalent formulations had no statistically significant association with myocarditis, unlike the monovalent original series [2].

Most cases resolve. A review found 95% of patients had rapid symptom resolution within days [4]. But the BMJ review noted that among 38 patients followed for 3 months, over half still had echocardiogram abnormalities or ongoing symptoms [6]. Deaths are rare but documented: the WHO global database recorded 741 deaths among 35,017 myocarditis reports linked to the COVID-19 mRNA vaccine [1].

My call: the myocarditis risk is real, concentrated in young males after the second dose, and higher with Moderna than Pfizer. The risk drops with boosters and bivalent formulations. Most cases resolve, but longer-term follow-up is thin. Confidence: moderate.