President Donald Trump has signed an order seeking to overhaul decades of federal guidance on childhood vaccines and urge states to rethink school vaccine mandates. Even changes to the federal vaccine schedule would require formal rulemaking through the CDC and its advisory committee. Doses of live viruses like those in the MMR vaccine are tiny — a fraction of a teaspoon. Trump's push to split the MMR vaccine stems from a long-held personal belief, rooted in a debunked theory linking childhood vaccines to autism. More than 40 studies involving over 5.6 million people have found no link between the MMR vaccine, or any vaccine, and autism.