The Food and Drug Administration is looking into whether deaths across multiple age groups are related to COVID-19 vaccines.
The announcement on Tuesday follows a November memo from a FDA official who claimed, without evidence, that COVID-19 vaccinations caused the deaths of 10 children.
The Department of Health and Human Services said a full investigation is now underway, but officials are not saying what data is being reviewed.
According to the World Health Organization, more than 700 million COVID-19 vaccine doses have been given in the United States.
Serious side effects remain rare.