The Pfizer-BioNTech coronavirus vaccine has been shown to be safe and highly effective in young children ages 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 1 in 5 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 The findings have not yet been peer-reviewed nor published in a scientific journal.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 multisystem inflammatory syndrome in children, or MIS-C. Still others may have lingering symptoms for months.

Nearly 30,000 children were hospitalized for COVID-19 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-19 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,” 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 ages 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, 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.”

The FDA’s full approval of the Pfizer-BioNTech vaccine in August did not include children ages 12 to 15, who are still getting the vaccine under emergency use authorization.