All 17 experts from the Advisory Committee for Immunization Practices who were replaced last month by Robert F. Kennedy Jr. called the health secretary's new picks "inexperienced and biased."

Though a strong promoter of vaccinations, Cassidy persuaded wary fellow Republicans to confirm Kennedy, President Donald Trump’s nominee to run the U.S. Department of Health & Human Services.

Although it has been largely phased out of U.S. vaccines, thimerosal has long been targeted by anti-vaccine advocates.