A new study has revealed that Pfizer’s COVID-19 vaccine may not be as effective in younger kids.

The New York Department of Health found the vaccine’s effectiveness in children ages 5 to 11 dropped from 68% to 12% over a two week period in December.

Officials say it is not clear exactly why.

Researchers say kids under 12 receive only one-third the dose size given to those 12 and up.