On Wednesday, Kennedy’s siblings Kathleen Kennedy Townsend and former congressman Joseph P. Kennedy II (D-Mass.), as well as niece Maeve Kennedy McKean, published an article in Politico Magazine accusing him of being “complicit” in a misinformation campaign.

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“Robert F. Kennedy Jr. … is part of this campaign to attack the institutions committed to reducing the tragedy of preventable infectious diseases,” they wrote. “He has helped to spread dangerous misinformation over social media and is complicit in sowing distrust of the science behind vaccines.”

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“We love Bobby,” they said, and praised his record on environmental issues. “However, on vaccines he is wrong.”

The family members point to the work that other members of the Kennedy family, including President John F. Kennedy, have done to promote public health. President Kennedy pushed for Americans to vaccinate their children, and in 1962 signed the Vaccination Assistance Act establishing the first nationwide programs for immunization.

But Robert F. Kennedy Jr. has taken a decidedly different path from his famous uncle. In 2005, he alleged in an article published in Rolling Stone and Salon that an ingredient called thimerosal, contained in some vaccines, was dangerous, and that the government was hiding its links to autism. It contained numerous factual errors, for which Rolling Stone had to issue several corrections. Salon ultimately retracted the story and removed it from its website.

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He has continued to write and give speeches on the repeatedly debunked claims that vaccines cause autism and leads a group called the Children’s Health Defense, which propagates misinformation about risks posed by vaccines.

In 2000, measles was declared eliminated from the United States, thanks to the country’s vaccination practices. But 764 measles cases across 23 states have been reported as of May 3, according to the latest CDC numbers.

The latest U.S. outbreaks are believed to be caused by travelers bringing the virus from countries having large measles outbreaks. When unvaccinated people are exposed, they are susceptible to becoming infected with the highly contagious virus. The disease can spread quickly within communities where not enough people are vaccinated to prevent an outbreak.

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Anti-vaccine sentiment has taken hold in several spots across the United States, creating dangerous conditions for the spread of the virus. Some communities in Washington state have low vaccination rates because of parents’ belief that the shots will harm their children. The Washington state legislature has passed a bill that would make it harder for parents to opt their children out of the measles, mumps, rubella vaccine. It is awaiting the signature of Gov. Jay Inslee (D).

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A vaccine misinformation campaign targeting Orthodox Jews in New York City is thought to have made the community vulnerable to the virus. There have been 466 confirmed cases of measles in the boroughs of Brooklyn and Queens since September, according to the city’s health department. Most of the cases have involved the Jewish community.

Last week, Health and Human Services Secretary Alex Azar said the outbreaks were posing an increasing public health risk.

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“We have come a long way in fighting infectious diseases in America, but we risk backsliding and seeing our families, neighbors, and communities needlessly suffer from preventable diseases,” Azar said.

In April, the Food and Drug Administration issued a stern and lengthy statement by Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, urging Americans to vaccinate.

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“We cannot state strongly enough,” Marks said, “The overwhelming scientific evidence shows that vaccines are among the most effective and safest interventions to both prevent individual illness and protect public health.”

Lena Sun contributed to this report.