The impact of anti-vaccine activists is spreading beyond humans. A recent study found many dog owners are skeptical of vaccinating their pets — even though that leaves animals and humans at risk.

The study, led by a researcher from Boston University’s School of Public Health and published in the journal Vaccine, found a sizable minority of dog owners have some hesitancy toward canine vaccines. Over one-third (37%) said they believed the shots were unsafe; 22% thought they were ineffective; and 30% thought they were unnecessary. Overall, 53% of dog owners endorsed at least one of these three misconceptions.

“My co-authors and I were stunned by how prevalent this phenomenon is,” lead author Dr. Matt Motta told CBS News.

Motta says an unvaccinated pet is a danger not just to other animals but also to the humans around them.

“If there are more unvaccinated dogs out there, the risk of disease transmission grows,” Motta said.