Kennedy Jr.'s nomination has been met with outrage from doctors, but it's not clear if he will disrupt immunization programs.

Vaccines are most effective at scale when enough people get them to provide "herd immunity" from a virus, doctors and officials say.

Plummeting trust in vaccines could be lethal on its own. But there are also ways Trump’s nominee for Health and Human Services could reduce vaccine availability, as well.

Consider three topics of much public discussion: measles vaccines, raw milk and water fluoridation. All three represent fault lines between what is said by public health agencies and by Mr. Kennedy and other skeptics. Where their messages differ is in the strength and complexity of the evidence.