The theory, however, has jumped species. It is increasingly being applied to pets in the United States and is gaining momentum in Britain — raising concerns that the already low vaccination rates in this country could fall further.
Those who fear vaccine side effects in their dogs claim the animals could develop canine autism, thyroid disease and arthritis.
Then, on Monday, the television show “Good Morning Britain” on ITV put out a call on Twitter to hear from dog owners who believed their pets showed symptoms of autism after receiving vaccinations, and from others who had stopped getting their pets vaccinated against dangerous diseases.
The next day, the veterinary association put out a statement on Twitter.
“We are aware of an increase in anti-vaccination pet owners in the U.S. who have voiced concerns that vaccinations may lead to their dogs developing autism-like behavior. There’s currently no reliable scientific evidence to indicate autism in dogs (or its link to vaccines),” the association said in its tweet.
It added: “Potential side effects of vaccines are rare and outweighed by the benefits in protecting against disease. BVA would be happy to provide evidence-based information on the issue.”
Many dog owners criticized the TV program for reporting what they called baseless anti-vaccine conspiracies. But others were intrigued: “I can’t believe I’m saying this but, how could you even tell your dog had autism?” one Twitter user asked.
The support for vaccinating pets was echoed by other agencies.
Britain’s independent Veterinary Products Committee, which reviewed all authorized dog and cat vaccines in the United Kingdom between 1999 and 2002, concluded that the “overall risk/benefit analysis strongly supports the continued use of vaccines.”
“It is extremely rare for any serious side effects to follow vaccinations,” the British Veterinary Medicines Directorate said in a statement. “Any adverse effect is generally far outweighed by the benefit of protection against serious disease.”
For a time, the anti-vaxxer movement in the United States gave rise to a public health crisis in at least 14 states, as outbreaks of measles, a disease that health officials had long declared beaten, reappeared in alarming numbers.
Fear of vaccines spread to Europe, and cases of measles rose in 2017, with the virus finding its way into areas with unvaccinated children from Romania to Britain. At least 35 children died of the disease in 2017, according to the World Health Organization.