Should I Vaccinate My Kids? Boston Doctors Talk Risk of Myocarditis

Children as young as 5 can now get Pfizer’s kid-size COVID-19 shot, but Moderna's may take a bit longer amid concerns about myocarditis, a rare but serious side effect.

The inflammatory heart condition already worried advisers to federal agencies in the process of approving the Pfizer vaccine in younger children and teenagers. Ultimately, health officials have come to the consensus that the vaccine’s benefits outweigh potential risks.

Doctors say COVID-19 infection can cause more serious heart inflammation.

The U.S. Centers for Disease Control and Prevention gave the final OK for youngsters age 5 to 11 to get kid-size doses of the vaccine made by Pfizer and its partner BioNTech late Tuesday. Pediatricians and other doctors' groups are now gearing up to help families decide whether to vaccinate their children.

In the weekly series, "COVID Q&A," NBC10 Boston asked three top Boston doctors on Tuesday for their thoughts on the COVID vaccine for younger children, including the risk of myocarditis.

Is myocarditis more likely with Pfizer or Moderna vaccines?

Myocarditis has been reported in younger men or teen boys who had both Pfizer and Moderna vaccines, according to the CDC. Cases typically arise within days of vaccination and subside with medicine and rest, the CDC said.

An official from the CDC's COVID-19 Response Team said that for 12- to 17-year-olds who received two doses of the Pfizer vaccine, the rate was as high as 56.7 cases per million doses in the three weeks following vaccination.

The U.S. Food and Drug Administration told Moderna last week that they need more time to study the rare risk of heart inflammation in young people before they consider approving the company's vaccine for 12- to 17-year-olds.

That review, which could last until January, will in turn delay emergency-use authorization of their lower dose of the vaccine for 6- to 11-year-olds, Dr. Shira Doron of Tufts Medical Center said.

"I think it's becoming quite clear that the rate of myocarditis with Moderna appears to be higher than the rate of myocarditis with Pfizer," Doron said. "And that's probably for the same reason that the antibody levels with Moderna appear to be higher and waning less than with Pfizer -- it has a higher dose."

In the U.S., the Moderna vaccine is authorized for people 18 and older. And in the meantime, reports from other countries have surfacing with varying levels of myocarditis rates from the Moderna vaccine, Doron said.

"So it makes sense, before the FDA provides that emergency-use authorization for 12- to 17-year-olds, to really understand what the real world data are suggesting that rate is so that they can be forthcoming with the public about what that risk is," Doron said.

What causes myocarditis?

One theory is that testosterone and puberty play a role. Testosterone affects the immune system, Dr. Daniel Kuritzkes of Brigham and Women's Hospital explained, as most sex steroids generally do, including estrogen.

"So when children go through puberty -- male children -- and begin making testosterone, that may have an effect on the immune system that allows it to respond differently to the vaccine than before," Kuritzkes said. "Then things settle down a little bit as you get older."

Although the risk of myocarditis increases with decreasing age, from 30 down to 16, the risk is actually lower than it is amongst older teens and young adults in that 12 to 15 year age group that is approved for Pfizer right now, Doron said.

"And when we talk about classic myocarditis, which is the heart information that comes from viruses themselves, it's also more common in post-pubertal males than in young children," Doron said. "And so there may be a testosterone effect, and what we are likely to find once it's rolled down in large numbers is that myocarditis may even be less common amongst children than it is among adolescents, teens and young adults, which would be obviously a very good thing, but also the dose is lower in the younger children."

Boston Medical Center's Dr. Davidson Hamer emphasized that the overall risk of myocarditis is low.

"On the myocarditis risk, it still looks like it's very low, and that many of the cases are relatively mild and transient and self resolving," Hamer said. "There are a lot of really anxious parents out there. I mean, I've been fielding a lot of questions from parents about, 'Should I immunize my 10-year-old? My 9-year-old?' And I've been saying, 'Yes, but you know, with the sort of limitation that we need larger numbers of children to be immunized to have a feel for what the risk of a rare complication like this is.'"