Selena Allison knew she wanted her baby vaccinated. It was just a question of when.

“I know vaccines work,” said the 34-year-old stay-at-home mom and Orange County, Calif., resident. “But they kind of freak me out. It’s kind of scary thinking about all the things going into your perfect newborn. I wasn’t comfortable giving five to six shots at a time to my tiny baby.”

Like a sizable and growing number of parents, Mrs. Allison devised a delayed schedule with her pediatrician for her now almost 5-year-old son. He got the MMR (measles, mumps and rubella) vaccine at age 3 rather than the recommended 12 to 15 months. By the time he started preschool at age 4, he had all his vaccines, according to federal recommendations.

Mrs. Allison was planning the same schedule for her 17-month-old son. But with measles striking so close to home—the recent outbreak began in Orange County—she had him get his first MMR shot Monday.

While parents who don’t vaccinate their children have been the focus of the recent measles outbreak, experts say vaccine delayers compose a larger and growing group. Their children are exposed to catching vaccine-preventable diseases at the most vulnerable ages and potentially threaten the concept of herd immunity which requires that the vast majority of the population is vaccinated to protect those that can’t get the vaccine, such as newborns and children with cancer or serious immune-system disorders.

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