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The protection offered by two doses of Pfizer’s COVID-19 vaccine in children declined during the Omicron wave, but a booster shot helped, suggests a new report published by the Centers for Disease Control and Prevention examining data from health care facilities across 10 states. 

The study’s authors chalked up much of the drop in protection to the Omicron variant. The effectiveness of two Pfizer shots in curbing the risk of E.R. or urgent care visits with COVID-19 declined to 46% among children ages 5 to 11; 45% among 12- to 15-year-olds, and 34% among 16- and 17-year-olds during the wave. 

Vaccine effectiveness was lowest among children 12 and older who were vaccinated at least five months earlier but had not received a booster shot; for them, the study found “no significant protection” during Omicron. But protection for children with a third dose was 81% during Omicron. 

“New CDC vaccine effectiveness studies and surveillance data show that COVID-19 vaccination in eligible children and adolescents continues to offer protection against severe COVID-19 disease, including during Omicron,” CDC spokesperson Kristen Nordlund said in a statement.

Additional data published Tuesday by the CDC, tracking rates of COVID-19 cases and deaths across 29 jurisdictions, also found the disease appeared less frequently among vaccinated children during the Omicron wave. 

While only nine COVID-19 associated deaths were reported between last April and the beginning of January in vaccinated children, Nordlund said, 121 had been reported in unvaccinated children. 

Similar findings were also released Monday by health authorities in New York state as a preprint, which has yet to be peer reviewed. The study found vaccine protection against infection “declined rapidly for children, particularly those 5-11 years” during the Omicron wave.

That study’s authors pointed to measures like spacing out the first and second doses by as much as 8 weeks, which the CDC recently floated, as one way to potentially boost protection for kids. 

Around a third of 5- to 11-year-olds and more than two-thirds of 12- to 17-year-olds have received at least one dose of COVID-19 vaccine, according to CDC’s latest tally. Since the agency green-lighted boosters for Americans as young as 12 in January, 21% of vaccinated adolescents have received an additional shot

“FDA has full confidence in the data that were used to support emergency use authorization of the Pfizer-BioNTech COVID-19 Vaccine in children 5 through 11 years of age and in the safety and effectiveness of the vaccine in preventing severe consequences from COVID-19, including hospitalization and death,” FDA spokesperson Abby Capobianco said in a statement. 

The new data echoes conclusions by Pfizer and BioNTech, which decided late last year to seek authorization of a “three dose vaccine approach for all ages.” Last month, the companies said they expected to have data from their clinical trials on three-dose protection available in early April. 

“The agency continues to work closely with the company as they accumulate additional data from their ongoing clinical trial, including on the potential need for an additional dose in children,” said Capboianco.