The vaccine prevented 98% of ICU visits and 94% of Covid-related hospitalizations in the real-world study of more than 1,000 adolescents ages 12 to 18 in 23 states published Wednesday in the New England Journal of Medicine.

While adolescents can develop severe Covid complications, it’s relatively rare that they do, making it harder to study vaccine efficacy than among older adults, and leading to some controversy about the use of the shots in younger people. For example, the trial data Pfizer submitted for authorization of its shot for 12- to 15-year-olds didn’t include enough cases to assess efficacy in preventing severe Covid. 

The research from the Centers for Disease Control and Prevention and a network of 31 hospitals is one of the most detailed yet showing that vaccines can prevent severe Covid complications in teenagers.  

The results show that “nearly all hospitalizations and deaths in this population could have been prevented by vaccination,” Vanderbilt University Medical Center pediatrics professor Kathryn Edwards wrote in an editorial accompanying the study.

The study compared 445 adolescents hospitalized with Covid with a control group of 777 admitted for other reasons, including those with Covid-like symptoms who tested negative. It ran from July 1, 2021 through Oct. 25, 2021, when the Pfizer shot was widely available for adolescents and the delta strain was dominant. Most of the kids hospitalized with Covid in the study were in the South, where the delta wave first hit the U.S.

Overall, the researchers found that 96% of the adolescents hospitalized with Covid and 99% of those who received life support had not been fully vaccinated. All seven patients who died of Covid and all of the 13 patients who received a last-ditch treatment called extracorporeal membrane oxygenation were unvaccinated, according to the findings. 

Some 74% of the adolescents hospitalized with Covid in the study had at least one underlying condition, such as obesity.