Pfizer's vaccine strongly protects teens against severe COVID-19 after two doses, CDC data suggests.
97% of US teens hospitalized with COVID-19 were unvaccinated in the cases the CDC studied.
Vaccines were "imperative" to reduce severe COVID-19 among young people, researchers said.
An analysis from the Centers for Disease Control and Prevention (CDC), published on Tuesday, found that full vaccination "reduced the risk for COVID-19 hospitalization in persons aged 12 to 18 years by 93%."
The study was one of the largest of its kind to pinpoint vaccine effectiveness against COVID-19 hospitalization in adolescents during a period where the highly infectious Delta variant was dominant, the study authors said.
It "reinforces the importance of vaccination to protect US youths against severe COVID-19," they said, adding that, as in-person school attendance increases, vaccines were "imperative" to reduce the incidence of severe COVID-19 among young people.
In early September, US pediatric hospital admissions reached their highest level since the start of the pandemic, the CDC's COVID-NET tracker shows.
As of Monday, 46% of US adolescents aged 12 to 15 years and 54% of those aged 16 to 17 years were fully vaccinated against COVID-19, according to the CDC. Pfizer's vaccine is currently the only shot in the US authorized for teens.
The researchers found that, in a study of 464 teens admitted to 19 hospitals across 16 states between June 1 and September 30, 179 had been hospitalized with COVID-19 and 285 didn't have the virus.
Nearly all, 97%, of those admitted with COVID-19 were unvaccinated, the study authors said. About two-thirds of those in the non-COVID-19 group hadn't had a shot, they said. Adolescents who had received one vaccine dose were excluded from the analysis.
To get to the 93% figure, the researchers used statistical methods to compare the likelihood of being vaccinated among teens hospitalized with COVID-19 compared to teens hospitalized without the virus.
In the group hospitalized with COVID-19, 77 of the 179, or 43%, required intensive care and two died, the study authors said. All of them were unvaccinated, they said, cautioning that vaccination status was sometimes self-reported so misclassification was possible in a few cases.
Most people in the study, 72%, had an underlying medical condition, such as obesity, the authors said. But the study was too small to estimate vaccine effectiveness according to specific medical conditions, they said.
The majority of teens, 61%, were from southern states due to high levels of COVID-19 in that region at the time. This "might limit the representativeness of the sample", the authors said.
It was also not possible to tell from the study how long vaccine protection lasts, they said.
Despite its limitations, the study adds to a growing body of research that suggests vaccines protect teens from the most severe effects of COVID-19.
A previous CDC study published on September 3 found that the COVID-19 hospitalization rate among unvaccinated adolescents was about 10 times higher than adolescents who were fully vaccinated against the coronavirus.
Late-stage trials of Pfizer's shot estimated that it had an efficacy of 100% against COVID-19 hospitalization in teens, but the numbers were small.
More recently, real-world data from Israel published in Emerging Infectious Disease on September 27 found Pfizer's vaccine was 91.5% effective against COVID-19 hospitalization in 12 to 15 year olds, but there weren't enough people in the study that caught COVID-19 to provide a precise estimate.
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