A new poll from The Washington Post shows that growing numbers of parents are delaying or skipping vaccines for their children, raising concerns among health experts.

The survey found that about one in six parents reported holding off on or skipping some of the standard recommended vaccines for kids.

Dr. Alok Patel, a physician and public health expert, warns that the trend could create serious risks. “That creates these unfortunate gaps in immunization and coverage which we know can leave kids vulnerable and possibly cause an outbreak if we don’t have that protection,” he said.

Parents who opted out of some vaccines cited concerns about side effects, doubts about safety, and a belief that not every recommended vaccine is necessary.

Health officials have long said maintaining high vaccination rates is critical to preventing outbreaks of diseases such as measles and whooping cough.