In the U.S., vaccines for the youngest are expected this fall.

Coronavirus vaccines may be available in the fall for U.S. children as young as 6 months, drugmakers say. Pfizer and Moderna are testing their vaccines in children under 12 years, and are expected to have results in hand for children aged 5 through 11 by September.

Compared with adults, children are much less likely to develop severe illness following infection with the coronavirus. But nearly four million children in the United States have tested positive for the virus since the start of the pandemic, according to the American Academy of Pediatrics.

Doctors continue to see rare cases of multisystem inflammatory syndrome in children, a condition linked to Covid-19 that can affect multiple organs, including the heart. Vaccinating children should further contribute to containment of the virus by decreasing its spread in communities.

Pfizer announced on Tuesday that it was moving to test its vaccine in children aged 5 through 12 years. It will begin testing the vaccine in infants as young as six months in the next few weeks.

The company hopes to apply to the Food and Drug Administration in September for emergency authorization of the vaccine for children ages 5 to 11. Results for children aged 2 through 5 could be available soon after that, according to Kit Longley, a spokesman for Pfizer.

Data from the trial for children between 6 months old and 2 years old could arrive in October or November, followed by a potential submission to the F.D.A. shortly thereafter, Mr. Longley added.

The Pfizer-BioNTech vaccine was authorized last month for use in children 12 through 15.

Based on data from an earlier study that assessed safety, Pfizer will give two doses of 10 micrograms each — a third of the dose given to adolescents and adults — to children ages 5 to 11 years, and two doses of three micrograms each to children 6 months to 5 years.

“We take a deliberate and careful approach to help us understand the safety and how well the vaccine can be tolerated in younger children,” said Dr. Bill Gruber, a senior vice president at Pfizer.

The study will enroll up to 4,500 children at more than 90 clinical sites in the United States, Finland, Poland and Spain. Pfizer’s researchers plan to submit the full data from the trials this summer for publication in a peer reviewed journal.

In March, Moderna began testing varying doses of its vaccine in younger children. That trial aimed to enroll 6,750 healthy children in the United States and Canada. Results are not expected till the end of the summer, and the vaccine’s authorization by the F.D.A. will take longer.

“I think it’s going to be early fall, just because we have to go down in age very slowly and carefully,” Moderna’s chief executive, Stéphane Bancel, said on Monday.

The company announced late last month that its vaccine was powerfully effective in 12- to 17-year-olds, and plans to apply to the F.D.A. for authorization in that age group. Last week, Moderna also asked the agency for full approval of its vaccine, rather than the emergency use for which it is currently authorized.

The United States will not be the first country in the world to authorize a coronavirus vaccine for young children. China has approved Sinovac’s vaccine for children as young as 3 years old, according to the company’s chairman. The approval has not been officially announced.