Struggling, inner-city parents are more likely to neglect completely vaccinating their children, while parents who refuse to vaccinate at all tend to be white and well-off, U.S. researchers reported on Tuesday.

The study is the first extensive national survey to look at why some children are not vaccinated and it shows a big difference between parents who are unable to get their children vaccinated, and those who are unwilling to do so.

By the time they are 3 years old, U.S. children are supposed to get at least 15 different shots. These include combined vaccinations for diphtheria, tetanus, and whooping cough, three doses of polio vaccine; one dose of combined measles, mumps, and rubella vaccine; three doses of Haemophilus influenzae type B vaccine; and others for hepatitis B and chickenpox.

In 2001, only an estimated 62.8 percent of all children aged 19 to 35 months were fully vaccinated, Philip Smith and colleagues at the Centers for Disease Control and Prevention’s National Immunization Program found.

More than 2 million children or 36.9 percent of toddlers were not fully vaccinated in 2001, and 17,000 children or 0.3 percent were not vaccinated at all, Smith’s team reports in Tuesday’s issue of the journal Pediatrics.

The CDC stressed that vaccines save lives. “Unvaccinated children are at increased risk of acquiring and transmitting vaccine-preventable diseases,” the team added.

They studied a sample of 150,000 children.

“Undervaccinated children tended to be black, to have a younger mother who was not married and did not have a college degree, to live in a household near the poverty level, and to live in a central city,” the researchers said.

“Unvaccinated children tended to be white, to have a mother who was married and had a college degree.”

The researchers found that “among parents of unvaccinated children, 47.5 percent expressed concerns regarding safety, compared with 5.1 percent of parents with undervaccinated children.”

And those who refuse vaccines often do not trust doctors.

“Among parents of unvaccinated children, 70.9 percent said that a doctor was not influential in shaping their vaccination decisions for their children, compared with 22.9 percent among undervaccinated children,” the researchers say.

As a result, there have been outbreaks of measles and polio in the United States, Smith’s team notes.

Of the children not vaccinated, 57 percent were boys.

“In response to concerns about the perceived risk of autism resulting from vaccinations, parents might have avoided having their sons vaccinated at a higher rate than their daughters, as a result of knowing that they have risk factors for autism and knowing that the rate of autism is 4 times greater for boys than for girls,” the researchers write.

Last month, the Institute of Medicine reported that a panel of experts could find no evidence that vaccines cause autism, but groups that question vaccine safety vowed to continue to fight to prove a link.