US President Donald Trump has signed an executive order directing federal officials to develop plans within 90 days to break up combination childhood vaccines — such as the measles, mumps and rubella (MMR) shot — into separate injections and space out their administration. The move would require pharmaceutical companies to revive individual vaccines not widely produced in the United States since the 1970s, construct new manufacturing facilities, and seek separate regulatory approval for each unbundled shot, a process experts say could take years and cost tens of millions of dollars. Public health specialists have condemned the order as scientifically unfounded, warning it would reduce vaccination rates by forcing families to make significantly more doctor visits, while drugmakers Merck and GSK have defended their existing combination vaccines and neither has announced plans to comply.