Sean O’Leary, chair of the Committee on Infectious Diseases at the American Academy of Pediatrics, studies vaccines and immunization for a living. And if you ask him to summarize what we know about vaccines, he’ll tell you, without hesitation, that vaccines work. “The science behind vaccines is very clear,” says O’Leary, a professor of pediatrics and infectious diseases at the University of Colorado School of Medicine and Children’s Hospital Colorado. “The benefits outweigh the risks.”