Pfizer added a third dose in the tests of its vaccine against covid-19 for children under 5 years

Pharmaceutical Pfizer reported Friday that it changed its plans and is testing adding a third dose of its vaccine against covid-19 in babies and children under 5 years of age, instead of the usual two doses.

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The addition of the third dose was decided after a preliminary analysis found that children between the ages of 2 and 4 did not have as strong an immune response as expected to the special low-dose injections.

Pfizer had planned to release the data from its study of children under 5 by the end of the year. It is not clear how the new decision will delay efforts to vaccinate younger children.

Pfizer and its partner BioNTech said that if the three-dose study is successful, they plan to apply for emergency authorization sometime in 2022.

A version of the Pfizer vaccine is now available for children between 5 and 11 years, whose dose is a third of that provided for people 12 years of age or older.

For children under 5 years of age, Pfizer is testing an even lower dose of just 3 micrograms – one-tenth the dose for adults.

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The scientists analyzed a group of children in the study for a month after the second dose, to determine if the little ones developed levels of antibodies similar to those of adolescents and adults who receive the regular doses.

Those younger than 2 years had similar levels of antibodies, but the level of immune response in children between the ages of 2 and 4 was lower, Kathrin Jansen, the head of vaccine studies at Pfizer, said during a teleconference Friday with investors.

For this reason, the study is being expanded to evaluate three injections of these low doses in children under 5 years of age. That third dose will be applied at least two months after the second.

The companies said that no safety-related issues were reported in the study.