The Pfizer / BioNTech COVID-19 vaccine is highly effective in 12- to 15-year-olds, the companies announced today in a press release. None of the children who received the vaccine in a clinical trial developed COVID-19, and they all developed strong antibody responses. There were no serious side effects.

The study included 2,260 participants. Around half received the two-dose vaccine, and the other half received placebo injections. There were 18 cases of COVID-19 found in the placebo group and none in the vaccine group.

Based on the study, the vaccine was 100 percent effective in this age group. The study was small, though, so it’s difficult to say if that number will hold up in a larger group. “But obviously, it looks good for the vaccine if there were zero Covid cases among the vaccinated people,” virologist Angela Rasmussen told The New York Times.

The 12- to 15-year-olds also had higher levels of antibodies one month after the second dose of the vaccine than people between 16 and 25 years old, the company said.

Right now, the Pfizer / BioNTech vaccine is authorized for people 16 years of age and older. The companies said they plan to ask the Food and Drug Administration to expand the authorization and allow the shots to be used for 12- to 15-year-olds as well. Albert Bourla, chief executive officer of Pfizer, said in a press release that the company hopes this age group can be vaccinated before the next school year.

Countries will need to vaccinate children and adolescents in order to reach herd immunity to protect communities against COVID-19. Although children rarely get seriously ill from the disease, they can still get sick and sometimes develop long-lasting symptoms. They can also contract and spread the virus.

Pfizer and BioNTech are also running a clinical trial testing the vaccine in children between six months and 11 years old. Moderna is testing its COVID-19 vaccine in adolescents between 12 and 17 years old and also in children between six months and 11 years old. That vaccine is currently authorized for people 18 and older. Johnson & Johnson, which also has a vaccine authorized for people 18 and older, has plans to test its one-shot immunization in younger people, from infants to 18-year-olds.

Over 96 million people in the United States have received at least one dose of a COVID-19 vaccine.