Although vaccination rates for U.S. children are high overall, many states are dropping below safety thresholds. That is because parents are opting out of state vaccination requirements for kids entering public school, despite a dearth of evidence that vaccines are harmful or unnecessary. “The vast majority of their concerns have no basis in science,” says William Schaffner, chair of preventive medicine at Vanderbilt University.

If enough parents in a community refuse or delay their children's vaccinations, an infectious disease can spread among many individuals. The outbreak can threaten all unvaccinated children, vaccinated children and adults who have weak immune systems, and babies who are too young to get their shots.

When vaccinations drop below the herd immunity threshold—the proportion of immune individuals needed to prevent widespread transmission—outbreaks rise. The 2012 U.S. outbreak of whooping cough (pertussis), which infected 42,000 people, was the largest since 1955. Rates for two crucial childhood vaccines, DTP (diphtheria, tetanus and pertussis) and MMR (measles, mumps and rubella), are falling (above). Other rates, such as for polio, are stable (not shown).

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