Health

Here’s Why Experts Are Comfortable With Myocarditis Numbers

The CDC’s vaccine advisors concluded on Wednesday that while the risk of myocarditis is real for young people following an mRNA COVID-19 vaccine — particularly after the second dose — the benefits far outweigh the risks.

The U.S. isn’t the only country to make that call. Not long after Israel announced that 275 myocarditis cases among 5 million vaccinated people were likely tied to Pfizer’s shot (mostly in men ages 16 to 30, and especially after the second dose), the country approved vaccinating kids 12 and up.

Here’s why experts are reassured by the numbers so far, particularly when it comes to children.

How often is myocarditis occurring after vaccination in young people?

During Wednesday’s Advisory Committee on Immunization Practices (ACIP) meeting, Tom Shimabukuro, MD, MPH, of the CDC’s Vaccine Safety Team, provided an updated analysis of data from the Vaccine Adverse Event Reporting System (VAERS).

Among people age 29 and under, there were 484 preliminary reports of myocarditis or pericarditis through June 11. Of these, 323 met the CDC’s working case definition of the condition, and 148 were still under review.

Of the 323 cases, 309 went to the hospital and the vast majority (295) were discharged; 79% had fully recovered, while nine remained hospitalized and two were in the ICU.

In a larger dataset involving all reported cases, myocarditis cases were indeed higher than expected, especially for males after dose 2, up to 21 days after injection:

  • Ages 12-17: 132 observed vs 1-12 expected
  • Ages 18-24: 233 observed vs 2-25 expected
  • Ages 25-29: 69 observed vs 2-21 expected
  • Ages 30-39: 71 observed vs 5-48 expected

Vaccine Safety Datalink findings in 12- to 39-year-olds revealed an overall myocarditis rate of 12.6 cases per million in the 3 weeks after the second dose, though most occurred within 5 days of vaccination, Shimabukuro reported.

He concluded that overall the condition was highly treatable and most patients recovered well.

That’s very different from when myocarditis is caused by a viral infection, Paul Offit, MD, of Children’s Hospital Philadelphia (CHOP), told STAT News. When myocarditis is caused by coxsackie or parvovirus, for instance, “it’s a serious disease, often involving ICU admission, and occasionally fatal. It can require a heart transplant. That’s not this. This is often transient, lasting 2 or 3 days, and resolves on its own for the most part or can be treated with anti-inflammatories.”

What about kids’ myocarditis risk after COVID or MIS-C?

It’s not clear exactly how often myocarditis occurs after COVID infection in kids, given high rates of asymptomatic disease in this population.

There are a bit more data on myocarditis in multisystem inflammatory syndrome in children (MIS-C), however — though only a fraction of kids with COVID get this disease. Estimates of myocarditis in patients with MIS-C have ranged from 23% to 60%, though a larger proportion (80% in one study) do have some form of cardiac involvement.

Even without myocarditis, MIS-C is a serious illness that frequently requires hospitalization, Offit said.

“MIS-C is a common cause of myocarditis,” he told STAT. “The cases we see at CHOP, when we see MIS-C they virtually all have myocarditis.”

What are the benefits of vaccination?

During the ACIP meeting, Megan Wallace, DrPH, MPH, and Sara Oliver, MD, provided a risk-benefit analysis of vaccination compared with myocarditis cases per million second doses over 120 days.

In males ages 12-17, vaccination prevents 5,700 COVID cases, 215 hospitalizations, 71 ICU admissions, and two deaths — at a cost of 56 to 69 myocarditis cases.

In males ages 18-24, vaccination prevents 12,000 COVID cases, 530 hospitalizations, 127 ICU admissions, and three deaths, at a cost of 45 to 56 myocarditis cases.

Higher vaccination levels can lead to less community transmission, Wallace and Oliver said, which can protect against the development and circulation of emerging variants — a big plus as the U.S. faces a growing prevalence of the new, more transmissible Delta variant.

What about skipping the second dose in kids?

The fact that myocarditis occurs more frequently after the second dose has prompted some parents to question whether the second dose should be dropped in kids.

Data from the pediatric Pfizer study, which involved about 2,400 kids (1,311 who got the vaccine and 1,129 who got placebo), showed that there were three cases of COVID-19 in the vaccine group compared with 12 in the placebo group 11 days after the first dose, for a vaccine efficacy of 75% — but the confidence intervals were very wide (95% CI 7.6%-95.5%).

Robert Schooley, MD, of the University of California San Diego, told MedPage Today via email that there’s no other literature about cutting the second dose in this population and that it would “make no sense from the standpoint of what we know of immunology. The concept of two doses is based on priming with the first dose and, with this baseline immunity present, getting an amplified response with the second dose.”

“One could argue that children have more robust immune systems and might have a slightly more vigorous immune response to the first vaccination than adults, and that if they do get infected a bit of immunity would be highly protective since children are more likely to do better clinically than adults,” Schooley said. “But with the variants on the side stage, I think we would be better suited to getting them fully vaccinated and giving them (and the population in general) a larger margin of protection.”

  • Kristina Fiore leads MedPage’s enterprise & investigative reporting team. She’s been a medical journalist for more than a decade and her work has been recognized by Barlett & Steele, AHCJ, SABEW, and others. Send story tips to [email protected] Follow

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