The Pfizer-BioNTech vaccine is highly effective at preventing COVID-19 hospitalization among adolescents, according to a study published Tuesday by the Centers for Disease Control and Prevention (CDC).

The two-dose Pfizer vaccine has been available since December in the U.S. to people age 16 and older, and to 12- to 15-year-olds since May. Pfizer’s clinical trial for the 12-15 group found the vaccine to be 100% effective at preventing outpatient COVID-19 hospitalization, but there has been limited data from real-world settings on adolescents.

Researchers examined the cases of 464 people ages 12 to 18 who were admitted to 19 pediatric hospitals in 16 different states from June 1 to Sept. 30. One hundred seventy-nine of the patients had COVID-19; the rest did not and were part of the control group.

The study found that 97% of the adolescents who were hospitalized with COVID-19 were unvaccinated. Out of the 77 adolescents who were admitted to intensive care units, 29 who were critically ill and required life support, and two who died from COVID-19, none were vaccinated.

Two Pfizer doses were 93% effective at preventing COVID-19 hospitalization during the time period studied, which was when the highly contagious delta variant was the predominant strain in the U.S., the researchers noted.

The study also found that the few vaccinated young people who did experience breakthrough cases requiring hospitalization recovered quicker — a median stay of three days compared to five for those not vaccinated.

The researchers wrote that their findings reinforce the importance of vaccinating young people against severe COVID-19. 

“These data suggest that increasing vaccination coverage among this group could reduce the incidence of severe COVID-19 in the United States,” the study says. “Further, as in-person school attendance increases, multicomponent preventive measures to reduce the incidence of severe COVID-19 among adolescents, including vaccination, are imperative.

Coronavirus hospitalizations among children surged this summer before hitting their highest point of the pandemic in early September, despite the availability of vaccines for those 12 and older.

Forty-six percent of U.S. adolescents 12-15 and 54% of 16- and 17-year-olds have been fully vaccinated.

A separate study published this month in the medical journal The Lancet found the Pfizer vaccine to be 81% effective in preventing COVID-19 hospitalization among 12- to 15-year-olds, but that evaluated a smaller sample size — just 45 cases. Another study out of Israel found the Pfizer vaccine to be 91.5% effective at preventing infection in kids 12-15, but it did not include enough cases to reach a conclusion on the shots’ effectiveness on hospitalizations.

Among the limitations acknowledged in the CDC study was that vaccinations in 12- to 15-year-olds only began in May, making it impossible to evaluate how long the vaccine’s protection might last.

The median age of the 464 patients in the study who were observed was 15. Seventy-two percent had at least one underlying condition, including obesity, and 68% attended in-person school.