In a trial, Pfizer-BioNTech’s low-dose shot fell short for children aged 2 to just under 5.

https://www.nytimes.com/live/2021/12/17/world/covid-omicron-vaccines#in-a-trial-pfizer-biontechs-low-dose-shot-did-not-provoke-an-adequate-immune-response-in-2-to-5-year-olds
Apoorva Mandavilli and Rebecca Robbins
New York Times
December 17, 2021