https://www.siliconvalley.com/2021/12/17/pfizer-tests-extra-covid-shot-for-kids-under-5-in-setback/

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.