At various times, Trump has alleged that giving children too many vaccines leads to autism. At a Republican debate last September, Trump said, "Just the other day, 2 years old, 2 and a half years old, a child, a beautiful child went to have the vaccine, and came back, and a week later got a tremendous fever, got very, very sick, now is autistic."
Trump has claimed that he's not against vaccines in general, but that giving too many vaccines at once leads to autism. Despite this claim, there is no evidence tying vaccination to autism.