The more people are vaccinated against a disease, the more trouble the disease has finding new hosts, and the better the chance it will die out. Many children—especially poor children—don't get all the vaccinations they should, but some children get no vaccinations at all. Since many parents now decide not to get their kids vaccinated, a new study from the Centers for Disease Control and Prevention looked at the characteristics of totally unvaccinated children to see how they're different from kids who just aren't getting as many vaccinations as they should.

What the researchers wanted to know: Who are these unvaccinated kids?

What they did: The National Immunization Survey is an annual survey of children between 19 and 35 months (about 18 months to nearly 3 years old). It dials random numbers to find households with a child in the right age range; when one is found, the interviewer asks for demographic information about the child, its mother, household income, and the child's vaccinations. If the person who answered the phone gives permission, the survey's organizers contact the children's doctor to get vaccination records. In this article, the researchers used the 2001 survey; children in that group should have been vaccinated for polio, measles/mumps/rubella, Haemophilus influenza type b (bacteria that can cause meningitis and other infections), hepatitis B, chicken pox, and diptheria/tetanus/pertussis (whooping cough), most of which require three or four shots.

What they found: Unvaccinated kids, compared with undervaccinated kids, were significantly more likely to be white, to have a married mother with a college degree, and to live in a household with an annual income over $75,000. They were also more likely to be male and to live in a household with four or more children. Undervaccinated kids were more likely than fully vaccinated kids to be black, to have a young, unmarried mother without a college degree, and to live near the poverty level. Interestingly, they were also likely to be in a household with four or more kids. In 2001, only about 63 percent of all children ages 19 to 35 months were fully vaccinated. Another 37 percent were undervaccinated, while an estimated 0.3 percent, or 17,000, were totally unvaccinated. Although all states require kids to get vaccinations before starting school, most states allow religious exemptions and 17 also allow exemptions for philosophical reasons.

What the study means to you: As (apparently unfounded) fears about vaccines causing autism, multiple sclerosis, and sudden infant death syndrome continue, there may be more outbreaks of diseases that used to kill many children. For example, a 2003 outbreak of whooping cough in Westchester County, N. Y., started in children whose parents had decided not to vaccinate them.

Caveats: Parents may have remembered their childrens' vaccinations inaccurately‑this could be checked if they gave permission to contact the doctor about vaccinations. Also, parents who refused to answer the survey might have been more likely to have unvaccinated children; their negative attitudes about vaccinations might have extended to vaccination surveys.

Find out more: The CDC's National Immunization Program includes recommends vaccinations for your kid. The website from the National Institute of Child Health & Human Development discusses the evidence for connections between vaccinations and autism.

Read the article: Smith, P.J., Chu, S.Y., and L.E. Barker. "Children Who Have Received No Vaccines: Who Are They and Where Do They Live?" Pediatrics. July 2004, Vol. 114, No. 1, pp. 187-195.