Childhood vaccines — and how and when to give them in combination — go through rigorous studies, and safety tracking continues for years as the shots are used. The order also includes several changes to the childhood vaccine schedule that his health department has already attempted, but that have been blocked by a federal judge. Even as the order calls for revised vaccine recommendations, states, not the federal government, have the authority to require vaccinations for schoolchildren. Changes to federal childhood vaccine recommendations customarily require approval from the director of the Centers for Disease Control and Prevention. They said that was Trump’s long-held view and that some countries provide individual measles vaccines in certain circumstances.