Why kids' COVID vaccines aren't performing like adults'

That said, Levy and the other experts I spoke with tend to more strongly implicate another culprit: the virus itself. Realistically, with kids’ trials running two doses of original-recipe vaccine during the Omicron era, the shots were probably never going to generate the knockout numbers that the adult shots did. There are now too many viral mutations in the picture; Moderna’s ballpark efficacy of 40 percent is “kind of what we would expect,” especially when transmission rates were as high as they were this past winter, Langel told me. It’s better, even, than the effectiveness of flu shots in years with vaccine-strain mismatches. In a press briefing last week, Anthony Fauci shared similar sentiments. Moderna’s new stats are “quite comparable,” he said, to what scientists have been seeing in other populations as of late. In adults, two mRNA doses just haven’t been as potent as three. All things considered, Fauci said, “the data looks pretty good.” It’s very possible that, were we to rerun studies in adults now, against Omicron, two full-size doses would struggle to get to 50 percent efficacy too.

Forty-ish percent efficacy against symptomatic illness may be about as good as we can get with two doses of mRNA vaccines without sacrificing safety. Maybe a bigger dose for kids would budge the numbers up, but “we have to consider the downside in terms of adverse events,” says Kathryn Edwards, a pediatrician and vaccine expert at Vanderbilt University. (Edwards is a former adviser to the FDA on vaccines, and sits on a safety-monitoring board for Pfizer’s shot.) Fevers in the littlest kids are an especially big concern because they can cause (self-resolving) seizures, so, even if rare, they could pose a major hurdle to clearing a new shot for use. Myocarditis, too, could be a problem: Moderna’s vaccine, which includes more mRNA in each injection, appears to have produced slightly higher rates of the rare heart-inflammation issue than Pfizer’s in young men. (No cases of myocarditis were picked up in Moderna’s under-6 trials.) “I don’t know that we need higher doses,” Creech told me. Rather, he and others think success will come down to the number of doses and their pacing. Vaccine makers could add a third or fourth injection or space the shots further apart, or both. They could even include an extra immune-system-tickling ingredient to rev the body’s defenses further.