Pfizer announced it was changing plans and testing three doses of its COVID-19 vaccine in babies and preschoolers after the usual two shots did not appear strong enough for some of the children. The change was announced after a preliminary analysis found 2 to 4 year olds did not have as strong an immune response as expected to the very low-dose shots Pfizer is testing in the youngest children.

Researchers analyzed a subset of youngsters in the study a month after their second dose to see if the kids developed levels of virus-fighting antibodies that were similar to teens and young adults who get the regular shots. Pfizer and its partner BioNTech said if the three-dose study is successful, they plan to apply for emergency authorization sometime in the first half of 2022. It’s disappointing news for families anxious to vaccinate their kids. Pfizer had expected data on how well the vaccines were working in children under 5 by year’s end, and it’s not clear how long the change will delay a final answer.

A kid-sized version of Pfizer’s vaccine already is available for 5- to 11-year-olds, one that’s a third of the dose given to everyone else 12 and older. For children younger than 5, Pfizer is testing an even smaller dose, just 3 micrograms or a tenth of the adult dose. The very low-dose shots appeared to work in youngsters under age 2, who produced similar antibody levels. But the immune response in 2- to 4-year-olds was lower than the study required, Pfizer vaccine research chief Kathrin Jansen said.

Rather than trying a higher-dose shot for the preschoolers, Pfizer decided to expand the study to evaluate three of the very low-dose shots in all the study participants , from 6 months up to age 5. That third shot will come at least two months after the youngsters’ second dose.
No safety concerns have been spotted in the study, the companies said.

Jansen cited other data showing a booster shot for people 16 and older restores strong protection, a jump in immunity that scientists hope also will help fend off the new omicron variant. The companies also are preparing to test a booster for 5- to 11-year-olds, who are just now getting their two-dose vaccinations. And they are testing different dose options for teen boosters. If the additional pediatric testing is successful, “we would have a consistent three-dose vaccine approach for all ages.”- Jansen said.