Federal health officials are examining the feasibility of taking aluminum salts out of vaccines, a prospect that vaccine experts said would wipe out about half of the nation’s supply of childhood inoculations and affect shots that protect against whooping cough, polio and deadly flu.

The review at the Food and Drug Administration began after President Trump listed aluminum in vaccines as harmful during a press briefing about the unproven link between Tylenol and autism.

Aluminum salts have been in vaccines since the 1920s and are added to enhance the immune-stimulating effect against the virus or bacteria covered by the inoculation. Robert F. Kennedy Jr., the nation’s health secretary, has been a longtime critic of aluminum in vaccines, which he has suggested is linked to autism.

Vaccine experts said the tiny amount of aluminum salts in vaccines — often measured in the one-millionth of a gram — has a long track record of safety and is essential to generating lasting immunity from disease. Developing vaccines without aluminum salts, they said, would require an entirely new formulation from scratch.

Such efforts take years of careful safety tests, cost hundreds of millions of dollars and could potentially expose thousands of infants to deadly diseases, given Mr. Kennedy’s insistence that new vaccines should be tested in humans against placebos.