Millions of adults in the US are rolling up their sleeves for one of three COVID-19 vaccines authorized in the US. With the safety and effectiveness of the shots proven in adults, the vaccine makers are testing them in teens, children, and infants.
The Moderna and Johnson & Johnson vaccines were authorized for ages 18 and up, while the vaccine from Pfizer and BioNTech was authorized for ages 16 and up. In January, Pfizer finished enrolling more than 2,200 teens aged 12–15 in a clinical trial of its vaccine. Moderna finished enrolling 3,000 teens aged 12–17 in its own study in February and is now launching a trial for 6,750 infants and children between 6 months and 11 years old. J&J is also making plans to test its vaccines in children.
About 23% of the US population—around 73 million people—is under 18, and many researchers think that getting kids vaccinated will be key to establishing herd immunity, ending the pandemic, and reopening society.
“It is extremely important to do these vaccine studies in children,” says Stephen Spector, a professor of pediatrics at the University of California San Diego School of Medicine, who is leading a trial of the Moderna vaccine. Herd immunity—the point at which enough people have achieved immunity to largely stop transmission—will require immunizing a minimum of 70–80% of the population, he says. “So in order to get herd immunity, we need to immunize children.”
At the same time, since children are the least likely to develop severe COVID-19, medical researchers say extra caution is warranted before authorizing the vaccines for them. As of March 17, the US Centers for Disease Control and Prevention’s weekly update reported that 226 people under 18 have died with COVID-19 in the country, compared with more than 417,000 people 65 and older and over 517,000 people overall.
“There is not the same degree of urgency for vaccinating children as in older adults; it is just a completely different calculation,” says H. Cody Meissner, chief of the Division of Pediatric Infectious Disease at Tufts Children’s Hospital. “I am certainly in favor of a vaccine for children and adolescents,” he says. “The question is, How thoroughly should the vaccine be evaluated in children before there is a broad authorization to administer it?”
Meissner is on the US Food and Drug Administration’s independent vaccine advisory committee, which votes on whether to authorize COVID-19 vaccines. He emphasizes that the shots are highly safe and effective in adults, but in December he abstained from voting to authorize the Pfizer-BioNTech vaccine because he didn’t think that Pfizer presented sufficient evidence of safety and efficacy of the shot in 16- and 17-year-olds.
Trials of the COVID-19 vaccines found that younger adults were more likely to have strong reactions than older adults, Meissner adds. “I think it is reasonable to reassure ourselves that these vaccines are equally safe in children as in adults.”
The safety of the vaccines will be top of mind in the trials for children, and finding the right dose may be key to minimizing side effects. In the first part of its new trial for kids under 12, Moderna is testing doses that are one-quarter, one-half, or equal to the dose currently administered to adults. After determining optimal doses for infants and children, Moderna will test those doses against placebo injections.
Since kids are much less likely to develop symptomatic COVID-19 than adults, it will be more difficult to prove that the vaccines are effective at preventing the disease. The vaccine makers plan to measure the levels of neutralizing antibodies, which prevent SARS-CoV-2 from infecting cells; if the levels are similar to those in adults who get the vaccine, the vaccines are likely to be effective in kids.
“It is a good way to evaluate the vaccine in children,” Spector says, even though scientists still haven’t determined the minimum level of antibodies a person needs in order to be protected.
Meissner expects a COVID-19 vaccine to be available for children eventually, but he doesn’t think the US should wait for kids to be vaccinated before reopening schools. “We hoped that by closing schools we would reduce transmission to adults,” he says. But scientists still don’t know how big a role children play in spreading the virus to adults, he adds. That makes it hard to know how important vaccinating children will be to achieving herd immunity. “So the question is, How much benefit will children get? Are we subjecting children to bear the real cost of an effort to control the pandemic?”