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The vaccine makers said new data shows children in that age group could benefit from another shot.
In a sub-analysis of 30 of the children, there was a 36-fold increase in neutralizing antibody titers compared to levels seen after two doses of the Pfizer-BioNTech COVID-19 vaccine.
“A robust response was observed regardless of prior SARS-CoV-2 infection,” the statement notes.
The data has not been published or vetted by independent experts.
Health officials already urge everyone 12 years and older to receive a booster dose and recently recommended a second booster for U.S. adults ages 50 years old and older.
Pfizer and BioNTech also plan to share the data with the European Medicines Agency (EMA) and other regulatory agencies around the world.
Pfizer tested the kid booster during the winter omicron surge.
Vaccinations are largely less effective against the variant than earlier versions of the coronavirus, but they still offer strong protection against severe disease.
Children ages 5 to 11 receive one-third of the dose given to those 12 and older.
Pfizer shots are only available to children in the U.S. and just over a quarter of the younger age group have gotten two doses.
The U.S. has not yet allowed the shots for children under 5.
The Associated Press contributed to this report.