Family quarrels are usually private things—unless of course, the family is famous.

A public spat among boldface names broke out on May 8, when three members of the Kennedy clan published a piece on Politico declaring that Robert F. Kennedy, Jr.—son of Bobby Kennedy—has been “tragically wrong” in his years-long crusade against vaccines, a crusade that seems especially irresponsible now as the country suffers through its worst measles outbreak since 1994. Kennedy has become a hero of the anti-vax crowd with his persistent claims that vaccines contain deadly ingredients, particularly a mercury-based preservative known as thimerosal, and that they are linked to autism.

He is wrong on both scores. No vaccines except some formulations of the flu vaccine contain thimerosal, and the type of mercury it uses is ethylmercury, which is cleared from the body quickly and harmlessly. And vaccines do not cause—and are not even associated with—autism. Full stop.

But RFK, Jr. persists, and so his siblings Kathleen Kennedy Townsend and Joseph P. Kennedy II, and his niece Maeve Kennedy McKean, sought to set him right. Kennedy, they wrote, “has helped to spread dangerous misinformation over social media and is complicit in sowing distrust of the science behind vaccines.”