Covid Vaccine Prompts Strong Immune Response in Younger Children, Pfizer Says

The Pfizer-BioNTech coronavirus vaccine has been shown to be safe and highly effective in young children aged 5 to 11 years, the companies announced early Monday morning. The news should help ease months of anxiety among parents and teachers about when children, and their close contacts, might be shielded from the coronavirus.

The need is urgent: Children now account for more than one in five new cases, and the highly contagious Delta variant has sent more children into hospitals and intensive care units in the past few weeks than at any other time in the pandemic.

Pfizer and BioNTech plan to apply to the Food and Drug Administration by the end of the month for authorization to use the vaccine in these children. If the regulatory review goes as smoothly as it did for older children and adults, millions of elementary school students could be inoculated before Halloween.

Trial results for children younger than 5 are not expected till the fourth quarter of this year at the earliest, according to Dr. Bill Gruber, a senior vice president at Pfizer and a pediatrician.

Pfizer and BioNTech announced the results in a statement that did not include detailed data from the trial. The findings have not yet been peer-reviewed nor published in a scientific journal.

But the new results dovetail with those seen in older children and in adults, experts said.

“There’s going to be a huge number of parents who are going to heave a big sigh of relief when they hear this,” said Dr. Kristin Oliver, a pediatrician and vaccine expert at Mount Sinai Hospital in New York. “We’ve been waiting for these kids to be protected.”

Children have a much lower risk of Covid-19 than adults, even when exposed to the Delta variant. Still, some small number of infected children develop a life-threatening condition called multi-system inflammatory syndrome in children, or MIS-C. Still others may have lingering symptoms for months.

Nearly 30,000 children were hospitalized for Covid in August; the least vaccinated states reported the highest rates. At Seattle Children’s hospital, about half of the children who are admitted for Covid are older than 12, according to Dr. Danielle Zerr, a pediatric infectious diseases expert at the hospital.

“I’ve been dismayed at the fact that the sickest children in our hospital with acute Covid-19 or MIS-C are children who could have been vaccinated,” Dr. Zerr said.

As ideological battles over masking and vaccine mandates play out in communities, the reopening of schools has fueled the surge. In Mississippi, among the states without a mask mandate, nearly 6,000 students tested positive for the virus in one week, and more than 30,000 students, teachers and staff had to be quarantined.

One county in South Carolina — where mask mandates are banned — had to quarantine more than 2,000 students in one day. Remote learning is not an option in many districts, so the safety of some medically vulnerable children in many parts of the country has become subject to the actions of others.

Unvaccinated children, even if they do not become ill themselves, can spread the virus to family members, teachers and others they interact with regularly — among them grandparents or those who are vulnerable to severe disease or death.

Mask wearing and good air circulation can significantly cut down virus transmission. But children are as likely as adults to transmit the virus to others, and more likely to do so than adults older than 60, according to a recent review of the evidence by the Centers for Disease Control and Prevention.

Pfizer’s trial included 2,268 children ages 5 to 11, two-thirds of whom received two doses of the vaccine three weeks apart; the rest were injected with two doses of saltwater placebo.

Given how rarely children become severely ill, the trial was not big enough to draw meaningful conclusions about the vaccine’s ability to prevent Covid or hospitalization. Instead, the researchers relied on measurements of the youngsters’ immune response, on the assumption that the protective levels of antibodies seen in older people would be as protective in younger children.

The children who got the vaccine produced a strong immune response, comparable to the levels of antibodies seen in the earlier trials of participants aged 16 to 25 years. But children in the 5- to 11-year-old group achieved this response with 10 micrograms of the vaccine, a third of the dose given to older children and adults.

At higher doses, the researchers observed more side effects in younger children, including fever, headache and fatigue, although none were severe, Dr. Gruber said. With the 10-microgram dose, “we’re actually seeing after the second dose, less fever, less chills than we see in the 16- to 25-year-olds.”

Immune defenses weaken with age, and the side effects also become milder. This decline in potency is the reason most vaccines are meted out in childhood — and why a much lower dose is often enough for children, said Dr. Yvonne Maldonado, who led the trial at Stanford University and chairs the American Academy of Pediatrics’ infectious disease committee.

“You want to hit the sweet spot, where you’re giving the lowest dose that might elicit reactions, but also high enough to get you a good, sustainable antibody response,” she said.

In children younger than 5, just three micrograms — a tenth of the adult dose — is being tested in trials and seems likely to prove sufficient, she said.

The F.D.A.’s full approval of the Pfizer-BioNTech vaccine in August did not include children aged 12 to 15, who are still getting the vaccine under emergency use authorization. As they did for adolescents, the companies will seek an emergency authorization for children aged 5 to 11.

Scientists at the F.D.A. must then weigh the benefits of the vaccine against the risk of side effects. In rare cases, the vaccine has led to myocarditis, an inflammation of the heart, in young people.

But a large Israeli study, based on electronic health records of two million people aged 16 and older, found that Covid is far more likely to cause these heart problems.

In order to detect side effects in younger children, the F.D.A. in July asked Pfizer-BioNtech and Moderna to expand their trials to include 3,000 children. But based on the company’s conversations with the F.D.A., Dr. Gruber said he believed the agency would greenlight the vaccine with the data available so far.

Discussions about the vaccine’s risks for children aged 6 months to 5 years are likely to be even more fraught than the vehement disagreements over immunizing healthy adults or teenagers.

“There’s some people out there who don’t really feel that there’s convincing data that under-fives need to be vaccinated,” Dr. Maldonado said.

Even though most children are spared severe illness following infection, pediatric hospitals and I.C.U. units are overflowing, she added: “Why wouldn’t you want to prevent an infection that could potentially put your child in the I.C.U.?”