Pfizer’s Covid vaccine is 93% effective at protecting against hospitalization in 12- to 18-year-olds, the Centers for Disease Control and Prevention said in a small study released Tuesday.

The CDC followed 464 Covid patients ages 12 to 18 spread across 19 U.S. pediatric hospitals from June through September when the delta variant was surging across the country. While roughly 72% of them had at least one underlying condition that increased their potential for severe symptoms, researchers found that 97% of those who ended up in the hospital weren’t vaccinated.

“These data suggest that increasing vaccination coverage among this group could reduce the incidence of severe COVID-19 in the United States,” CDC researchers wrote in their Morbidity and Mortality Weekly Report.

Among the Covid patients, 6 were vaccinated and 173 were unvaccinated. Some 43% required intensive care, and 16% of the critically ill children received life support, with two deaths among the group, according to the study.

The CDC’s findings are similar to the results of a study conducted in Israel, which found that Pfizer’s Covid vaccine was almost 92% effective in preventing hospitalization among 12- to 15-year-old patients. But the Israeli study did not feature enough cases to properly gauge the vaccine’s full effectiveness against Covid hospitalizations, the CDC wrote.

Some 61% of the study’s participants were from the south, the CDC said, since the region experienced elevated levels of Covid transmission from June through September. The high concentration of southern patients also could’ve affected the findings, researchers said.

The Food and Drug Administration approved Pfizer’s Covid vaccine for anyone over 16 on Aug. 23, leaving it on emergency use status for children 12 to 15, pending further review. More than 104 million people in the U.S. have been fully vaccinated with Pfizer’s vaccines, while over 9 million have already received a Pfizer booster dose, the CDC reported as of Tuesday.

The CDC authorized Pfizer’s boosters for select at-risk groups last month, including anyone 65 and older, all medically vulnerable adults and those who face exposure to Covid due to their work. U.S. health leaders have refrained from approving boosters for those 12 to 18, citing their strong likelihood for surviving Covid and concerns over the risk of two vaccine-induced rare heart inflammation conditions, myocarditis and pericarditis.

Researchers noted that the study was limited by its small sample size, which prevented them from properly measuring vaccine effectiveness in patients with underlying conditions. They added that they also could not determine the vaccine’s effectiveness against different Covid variants and said some participants may have misrepresented their self-reported vaccination status.

Pfizer is currently waiting for the Food and Drug Administration to authorize its shots for children 5 to 11. The company released data in September that indicated its two-dose vaccine regimen yielded a “robust” immune response among the younger pediatric age group, and the FDA could clear the shots later this month.

The CDC reported that 46% of 12- to 15-year-olds in the U.S. were fully vaccinated against Covid, while 54% of 16- to 17-year olds had received a complete series of doses as of Monday.