When will children under age 5 be able to get the COVID vaccine?

The omicron variant and the winter wave of COVID-19 have people wondering about those who are unvaccinated—and not by choice.

They are children under age 5.

Pfizer and BioNTech announced today that they are going to test adding a third shot of the COVID-19 vaccine to the protocol for children ages 2 to 4, because two doses failed to cause an adequate immune response. Two doses were adequate for kids ages 6 to 24 months, though.

If the three-dose study works, the companies say they’ll send the data to the U.S. Food and Drug Administration (FDA) as part of their application for an emergency use authorization in the first half of 2022. That’s a delayed timeline compared to a more optimistic estimate earlier in December from Pfizer CEO Albert Bourla, who told NBC News that data might be available by the end of this year.

Pfizer and BioNTech will now examine the effectiveness of a third dose of a 3-microgram injection a minimum of two months after young children get their second ones.

Currently, kids under 5 are the only group not authorized for the COVID-19 vaccination.

The drugmakers’ Phase 1/2/3 trial initially enrolled as many as 4,500 children, between 6 months and 12 years old in the United States, Finland, Poland, and Spain.