Vaccine opponents, often the subject of ridicule, have found fresh energy in the election of a president who has repeated discredited claims linking childhood immunizations to autism and who has apparently decided to pursue them. With President Trump’s support, this fringe movement could win official recognition, threatening lives and making it urgent that health officials, educators and others respond with a science-based defense of vaccines.

Vaccines have saved lives by protecting children and adults from diseases like measles, polio, smallpox, cervical cancer and whooping cough. And there is no evidence whatsoever that vaccines or a preservative used in flu shots cause autism. Scientists have also shown that parents who refuse to immunize their children are threatening to undo decades of public health gains.

Yet, activists like Robert Kennedy Jr. continue to push pseudoscience about immunizations. The terrifying thing is that they appear to have Mr. Trump's ear. After a meeting with the president last month, Mr. Kennedy said that the president would name him to head a new committee on vaccine safety; the government already has an advisory group that is meeting this week. And last week, during a news conference with Robert De Niro, Mr. Kennedy offered a $100,000 reward to anyone who could prove that vaccines are safe for children and pregnant women.

Of course, countless studies show that vaccines are safe and effective — more than 350 health groups compiled a list for Mr. Trump — but they haven’t penetrated the reality distortion field created by Mr. Kennedy and his fellow travelers. The biggest danger is that their movement will sow enough doubt that more parents will refuse to let their children be immunized. In some states like Texas the number of children who do not receive vaccines has risen sharply over the last decade and is now in the tens of thousands. Not only does this increase the risk of infections to the youths who don’t get shots, but it also threatens infants who are too young and children who cannot be vaccinated for medical reasons. This phenomenon has already led to outbreaks of measles and mumps.