The trial included 2,260 adolescents ages 12 to 15. The children received two doses of the vaccine three weeks apart — the same amounts and schedule used for adults — or a placebo of saltwater.
The researchers recorded 18 cases of coronavirus infection in the placebo group, and none among the children who received the vaccine. Still, the low number of infections makes it difficult to be too specific about the vaccine’s efficacy in the population at large, Dr. Rasmussen said.
“But obviously, it looks good for the vaccine if there were zero Covid cases among the vaccinated people,” she added.
The adolescents who got the vaccine produced much higher levels of antibodies on average, compared with participants 16 to 25 years of age in an earlier trial. The children experienced the same minor side effects as older participants, although the companies declined to be more specific.
Dr. Iwasaki said she had expected antibody levels in adolescents to be comparable to those in young adults. “But they’re getting even better levels from the vaccines,” she said. “That’s really incredible.”
She and other experts cautioned that the vaccine might be less effective in children, and adults, against some of the variants that have begun circulating in the United States.
Pfizer and BioNTech have begun a clinical trial of the vaccine in children under 12 and started inoculations of children ages 5 to 11 just last week. Company scientists plan to start testing the vaccine next week in even younger children, ages 2 to 5, followed by trials in children ages 6 months to 2 years.