Pfizer said Friday it was changing plans and testing three doses of its COVID-19 vaccine in babies and preschoolers after the usual two shots didn't appear strong enough for some of the children.

Pfizer announced the change after a preliminary analysis found 2- to 4-year-olds didn't have as strong an immune response as expected to the very low-dose shots the company is testing in the youngest children.

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It's disappointing news for families anxious to vaccinate their tots. 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.

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.

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.

Researchers analyzed a subset of youngsters in the study a month after their second dose to see if the tots developed levels of virus-fighting antibodies that were similar to teens and young adults who get the regular shots.

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 Friday in a call with investors.

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.

Jansen said if the additional pediatric testing is successful, “we would have a consistent three-dose vaccine approach for all ages.”