A vial of the Pfizer-BioNTech COVID-19 vaccine at a vaccination clinic in the gymnasium of Antioch Middle School in Antioch, Calif., on May 19, 2021. (Eli Walsh/Bay City News)

The U.S. Food and Drug Administration updated its approval Tuesday of the Pfizer-BioNTech booster vaccine, making it available to some children under age 5.

Before this update, children under 5 were not eligible for COVID-19 booster shots. Instead, they received three doses of the regular vaccine.

As of December 2022, children age 4 and younger who have not been vaccinated receive the omicron variant-specific booster vaccine as the third dose in their primary vaccine series, following two doses of the original Pfizer vaccine.

However, children in that age range who completed their initial vaccination series before December 2022 only received three doses of the original Pfizer vaccine, and are less protected against more infectious variants of the virus as a result.

FDA officials updated the vaccine’s emergency use authorization Tuesday to allow those children who only received the original Pfizer COVID vaccine to receive one dose of the bivalent booster if it has been at least two months since they completed their initial series.

Other children under age 5 are not eligible for the booster, although everyone age 5 and up is eligible for a booster.

“Currently available data show that vaccination remains the best defense against severe disease, hospitalization and death caused by COVID-19 across all age groups, and we encourage all eligible individuals to make sure that their vaccinations are up to date with a bivalent COVID-19 vaccine,” said Dr. Peter Marks, the director of the FDA’s Center for Biologics Evaluation and Research.

Clinical data has found that both the original Pfizer vaccine and the booster vaccine that targets the omicron subvariants BA.4 and BA.5 are safe for everyone aged 6 months and up and effective at preventing the worst outcomes of COVID infection, including serious illness and death.

COVID vaccines are available at primary care providers, retail pharmacies and some facilities operated by local health departments.