This week the New York State Department of Health and University at Albany School of Public Health released the findings of a study conducted to

estimate BNT162b2 [Pfizer] vaccine effectiveness against COVID cases and hospitalizations among children 5-11 years…during December, 2021 and January, 2022.

Data from December 13, 2021 to January 30, 2022 showed that vaccine effectiveness among the state’s fully vaccinated children aged 5-11 dropped precipitously from 68% to just 12%.

Table 1 from N.Y. Dept. of Health Study

The report concedes that

In the Omicron era, the effectiveness against cases of [the Pfizer vaccine] declined rapidly for children, particularly those 5-11 years.

The study suggests that the drastic decline in vaccine efficacy for children 11 years old and younger is likely due to the reduced dose size that they receive.