Why Kids Might Be Key to Reaching Herd Immunity

A pair of hands in blue gloves administer a vaccine to a young person's arm.
REUTERS/Flavio Lo Sc

A few days after Christmas, Molly Hering, 14, and her brother, Sam, 12, got their first shots as part of the Pfizer COVID-19 vaccine trials for kids. Their mom had heard about a clinical trial being conducted at Cincinnati Children’s Hospital, and Molly told me that she’d agreed to join because she wanted to contribute to the vaccine-development effort.

Molly and Sam’s dad was recently hospitalized with COVID-19. (He recovered.) Both kids have spent most of the past year dealing with Zoom school and its attendant technical glitches. Molly finally went back to in-person ninth grade this month, but masks and social distancing are required at school. Like everyone else, she’s looking forward to the end of the pandemic. “I’ll finally be able to go to school normally,” she said.

With COVID-19 vaccines proven to be safe and effective in most adults, Pfizer and Moderna have both begun U.S. trials for kids as young as 12. And if those trials go smoothly, the vaccines will be tested in younger and younger kids. This is typical for new vaccines: “It’s called the age deescalation strategy,” Carol Kao, a pediatrician at Washington University in St. Louis, told me.

There are some 70 million kids in the U.S., nearly a quarter of the country’s population. Children in general are not especially vulnerable to COVID-19; most infections are mild or even asymptomatic. In some very rare cases—fewer than 0.01 percent—young patients can develop a complication called multisystem inflammatory syndrome, or MIS-C, but it is generally quite treatable in a hospital.

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Vaccinating kids, however, is often not just about the direct and immediate benefits to them. It’s also meant to protect children against diseases that would otherwise become more dangerous for them as adults—measles, mumps, and chicken pox are three common examples—and dampen the overall spread of these diseases. In the short term, the primary reason to vaccinate children against COVID-19 may be that the U.S. will have a hard time reaching herd immunity otherwise.


Vaccines that work in adults generally work in children. But their effects can differ, especially in very young children. In newborns, for example, antibodies passed to them in utero can interfere with the protection conferred by the measles vaccine, which is why that vaccine is not given until babies are 12 to 15 months old. An early version of the pneumococcal vaccine did not work well in children under 2, because it stimulated a part of the immune system that was not yet mature.

Multiple factors determine the recommended age for a vaccination. “For example, when’s the peak incidence of disease? When is a child most likely to respond to the vaccine?” says Cody Meissner, an expert on pediatric infectious diseases at Tufts. The answers to those questions might not align. For instance, the vaccine for HPV, a sexually transmitted virus that can lead to cervical cancer, is given to boys and girls as young as 9 years old because it stimulates a better immune response in preteens than in older adolescents, even though preteens are unlikely to need the protection until later in life.

Even though kids rarely get seriously sick from COVID-19, the vaccine can protect them from an illness that may still be bad enough for them to miss school and their parents to miss work, Jeff Gerber, a pediatrician at Children’s Hospital of Philadelphia, told me. “Even those two-to-three-day illnesses can pile up.” He pointed out that the flu vaccine is recommended for kids, and about the same number of children died of the flu last season as have died of COVID-19 to date.

But the main argument for broadly vaccinating children is that doing so is likely to reduce COVID-19 transmission. Although schools have not been sources of large outbreaks, many switched to distance learning, and most of those that held in-person classes required masks and distancing. If school buildings reopen without these precautions sometime this year, after adults get vaccinated but while kids are still vulnerable, they will essentially be hosting mass gatherings of unvaccinated people, says Jason Newland, a pediatrician at Washington University. “Guess who’s going to end up having it? All the kids,” he told me. “And those kids with certain underlying conditions are disproportionately impacted.” What’s more, kids could bring COVID-19 home from school, even if they don’t have symptoms. “Children could pass it on to Grandma and Grandpa. They can pass it on to another loved one who has diabetes or has obesity or has chronic kidney disease” and is not yet vaccinated, Newland said.

Public-health experts think that if precautions are in place, community transmission is low, and teachers and high-risk people are vaccinated, reopening schools in 2021 will be worth any remaining risk, given the many and wide-ranging consequences of keeping them closed. But the more children are vaccinated, the safer and more normal school reopenings will be.  

The ultimate goal of most vaccination campaigns is not just to protect the individuals who get the vaccines, but to prevent the spread of the disease to those who can’t get it, such as infants and people at risk of allergic reactions. The more transmissible the virus—and unfortunately COVID-19 appears to be evolving to be more transmissible—the more people need to be vaccinated in order to reach herd immunity and stop its spread through a population. The herd-immunity threshold against COVID-19 is estimated to be somewhere between 60 and 90 percent of the population. Because some people can’t get the vaccine and some may be steadfastly opposed to it, the U.S. needs to vaccinate as many of the rest of us as it can. “What we want to do is reduce the number of people who might be contagious to others,” Meissner told me. Children will almost certainly need to be a part of that effort.

A still unanswered question about the COVID-19 vaccines, however, is how well they protect against asymptomatic transmission of the virus. When vaccinated people are exposed to the virus, their bodies’ immune response tamps down its replication—enough to reduce symptoms by 95 percent, according to the results of the Pfizer and Moderna trials for adults. But a person who is vaccinated and then exposed might still carry enough virus to spread it to others. Experts think the vaccines very likely reduce the risk of asymptomatic transmission, but follow-up studies are needed to find out by how much.


How soon the vaccines are available to children will depend on how long the clinical trials and the FDA review process take. In the most optimistic scenario, a vaccine could be available for large numbers of kids, especially older ones, in time for the start of the school year in the fall. But last week, Operation Warp Speed said that Moderna was having trouble recruiting enough participants in its trial for adolescents, having enrolled only 800 out of a planned 3,000. The company’s CEO also said that while Moderna would soon begin trials for kids ages 1 to 11, it did not expect to have results until 2022. Pfizer’s adolescent trial has reportedly finished enrollment, though the company declined to specify when it plans to move on to younger children.

The FDA fast-tracked the COVID-19 vaccines for adults using a process called emergency use authorization. But the normal vaccine-approval process may be more appropriate for children, says Vanderbilt’s Tina Hartert, who is leading a study on the incidence of COVID-19 in kids. Approval will take more time for several reasons. For example, the FDA has said that it wants to see more safety data—six months or more depending on the novelty of the vaccine technology—compared with the two months required for emergency use.

The newness of the COVID-19 vaccines combined with the mildness of the disease in children might lead to hesitancy in some parents. When the chicken pox vaccine first became available, in the 1990s, uptake was slow—until public schools started requiring it. Meissner, who sits on the FDA’s vaccine advisory committee, said he thinks schools are unlikely to require a vaccine that is authorized only for emergency use. But there is genuine debate among advisory groups about whether to mandate COVID-19 vaccination at all. “Once you require a vaccine,” Meissner says, “a lot of people who ordinarily might get the vaccine become indignant and say, ‘I’m not going to be told what I have to do for myself or my family.’”

In the long term, the COVID-19 virus is unlikely to go away entirely—and neither is the need for vaccination. If the virus continues to mutate or if immunity wanes, annual vaccinations or boosters every few years might be necessary. The vaccines could also become part of the recommended childhood immunization schedule. Experts say this is the best way to ensure that the entire population remains protected. “The most successful implementation programs are universal pediatric programs,” Kathleen Neuzil, a vaccine researcher at the University of Maryland School of Medicine, told me. Parents are already used to their children getting shots from pediatricians, and pediatricians are already used to vaccinating large numbers of children. Although children may not be particularly vulnerable to COVID-19, they will be when they reach adulthood. And vaccinating large numbers of adults, as the world is currently finding, is very difficult indeed.

Molly Hering, who is participating in the Pfizer trial, doesn’t yet know whether she got the placebo or the actual vaccine. She and her brother are using an app to track their side effects; she had some nausea and headaches after she first got the shot, while he had a low fever. They are going back for their second shots this week. For childhood vaccinations to help slow the spread of the virus, more volunteers will have to join them in the trials.