In November 2019, when an epidemic of measles was killing children and babies in Samoa, Robert F. Kennedy Jr. — who in recent days became Donald Trump’s pick to lead the department of Health and Human Services — sent the prime minister of Samoa at the time a four-page letter. In it, he suggested the measles vaccine itself may have caused the outbreak.

He claimed that the vaccine might have “failed to produce antibodies” in vaccinated mothers sufficient to provide infants with immunity, that it perhaps provoked “the evolution of more virulent measles strains” and that children who received the vaccine may have inadvertently spread the virus to other children. “Please do not hesitate to contact me if I can be of any assistance,” he added, writing in his role as the chairman of Children’s Health Defense, an anti-vaccine group.

At the time of his letter, 16 people, many of them younger than 2, were already reported dead. Measles, which is among the most contagious diseases, can sometimes lead to brain swelling, pneumonia and death. For months, families grieved over heartbreaking little coffins, until a door-to-door vaccination campaign brought the calamity to a close. The final number of fatalities topped 80.

I was in Samoa during that outbreak as part of my more than 16 years of reporting on the anti-vaccine movement. The cause of the outbreak was not the vaccine, but most likely an infected traveler who brought the virus from New Zealand, which that year had seen the biggest measles outbreaks in decades, especially among that country’s Indigenous and Pacific Islander communities. Migration and poverty were likely factors in a sudden spread of measles in Samoa and New Zealand. But, as an editorial in the New Zealand Medical Journal reported, so too was a factor that Mr. Kennedy specializes in: “Increasing circulation of misinformation leading to distrust and reduced vaccination uptake.” Samoa’s vaccination rates had fallen to less than a third of eligible 1-year-olds.

Vaccine skepticism has ballooned worldwide, and Mr. Kennedy and others who back him have encouraged it. Americans may be well aware that their possible future health leader holds dangerous beliefs about vaccines. The consequences of his views — and those of his orbit — are not merely absurd but tragic.

In my reporting, parents have mentioned fearing vaccines after watching “Vaxxed,” a 90-minute documentary, which had also toured countries such as New Zealand. The film, focused on unproven allegations, was released more than three years before the Samoa measles outbreak. Among much else, it claimed the Centers for Disease Control and Prevention had committed fraud.