It’s not too late for Senate Republicans to begin correcting the worst mistake they’ve made this year: confirming Robert F. Kennedy Jr. as secretary of Health and Human Services. In just a few months, Kennedy has helped bring a pox upon the country — and until Republicans get serious about holding him accountable, more Americans will die, and the president’s legacy on health and safety will be badly tarnished.

Kennedy, who has no training in medicine or health, has long been the nation’s foremost peddler of junk science and the crackpot conspiracy theories that flow from it. The greatest danger in elevating him to HHS secretary was always that he would use his position to undermine public confidence in vaccines, which would lead to needless suffering and even death. And so it has come to pass.

In 2000, the Centers for Disease Control and Prevention declared that measles had been effectively eliminated in the US, thanks to vaccine rates that hovered around 95%, the level needed for herd immunity. Now, in no small part because of the doubt Kennedy has been sowing about the safety of........

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