The study, of about 200 parents, found that those who had opted not to follow the standard vaccine schedule often sought advice from anti-vaccine friends and family.
Experts said it's not certain that the advice actually steered parents in an anti-vaccine direction: Parents who were already prone to shunning vaccines may have turned to like-minded people for reinforcement.
"It's the chicken-and-egg question," said researcher Emily Brunson, an assistant professor of anthropology at Texas State University, in San Marcos. "The answer is, we don't know which came first."
To be more sure, Brunson said, parents would have to be followed over time, to see whether undecided parents actually base vaccine decisions on advice from other people.
But Brunson said she thinks family, friends and others in parents' "social networks" really are an important influence.
Dr. Douglas Opel, of Seattle Children's Research Institute and the University of Washington, agreed.
"It is unclear how these groups influence parents. Do they simply reinforce the vaccine decisions parents would have made otherwise, or do they actually function as a way that provokes a parent to consider other ideas?" said Opel, who wrote an editorial published with the study, which appears online April 15 and in the May print issue of the journal Pediatrics
Opel said his hunch is that family and friends reinforce parents' existing views. But even if that's true, they are still a big influence by bolstering parents' beliefs.
Experts recommend that babies and young children routinely receive vaccinations against a host of common (or once common) infectious diseases, such as measles, mumps, whooping cough, chickenpox and hepatitis.
But some parents balk at those recommendations, largely because of a purported link between vaccines and autism. More than a decade's worth of studies have failed to confirm that link exists, but anxiety remains: A recent study of U.S. parents found that about one-third thought children receive too many vaccinations in their first two years, and they thought the shots could contribute to autism.
Brunson wanted to see where parents are turning to get their information, so she recruited nearly 200 parents of children 18 months old or younger. About 130 had their child up to date on all vaccines (and were dubbed "conformers") and 70 had opted to skip or delay at least some vaccinations ("nonconformers").
In an online survey, Brunson asked the parents to list the people and other sources -- such as websites and books -- they had gone to for vaccine advice.
She found that nearly all parents had sought advice from other people -- usually several people, including their doctor, spouse, family members and friends. And parents' ultimate decisions generally fell in line with that advice.
Among nonconforming parents, nearly three-quarters of their social circle recommended not vaccinating, on average. That was in sharp contrast to the conformers, whose social circles by and large said they should have their child vaccinated on time.
Brunson found that the more anti-vaccine views parents heard from their circle, the more likely they were to skip or delay vaccinations. And people seemed to matter more than information sources, such as the media.
She noted that the media often "gets a bad rap" as being a well of vaccine misinformation. But in this study, nonconforming parents actually got a more positive view of vaccines from the media than they did from their social circles.
Brunson and Opel said the findings speak to the power of the people in our lives.
"Parents do not make immunization decisions in a vacuum," Opel said. "Parents listen to and are influenced by other parents."
He said parents who vaccinate might try being more "vocal" to other parents about why they made their decision.
Brunson said efforts to encourage parents to vaccinate often focus on the role of pediatricians. "But this study is saying that we probably need to have a much broader approach than that," she said.
Media campaigns and other approaches that reach the general public, not just parents, might work better, Brunson said.