Public health experts have been quick to condemn an executive order from President Donald Trump aimed at upending childhood vaccinations in the U.S., but the biggest obstacles may be the unprecedented financial and logistical challenges it would impose on parents, health providers and drugmakers.

Monday’s announcement by the Republican president calls for separating combination shots — including the measles, mumps and rubella, or MMR, vaccine — into separate injections. Appointments for that and other vaccinations should be spaced out whenever possible, the order states.

To accomplish that, drugmakers would need to revive a slate of individual vaccines that have not been widely used in the U.S. for decades. They would also have to build new manufacturing plants capable of producing millions more vaccine doses than the nation currently uses.