Jim O’Neill, acting director of the Centers for Disease Control and Prevention, called for drastic changes to the measles, mumps, and rubella vaccine that aren’t supported by medical research, intensifying the Trump administration’s criticism of a shot that’s a cornerstone of the childhood vaccination schedule.

“I call on vaccine manufacturers to develop safe monovalent vaccines to replace the combined MMR and ‘break up the MMR shot into three totally separate shots’,” O’Neill said in a post on X on Monday. His post did not give a reason for breaking up the MMR shot, but linked to a message from President Trump, which called for separating the shot into three vaccines and giving it at different medical appointments.

Outside experts have previously said that the idea could leave childre

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