Family physicians and the government recommend that babies receive as many as 20 vaccinations between birth and 18 months. That's a lot of shots. Even though some are given in combos (like measles/mumps/rubella), the abundance of needles means that it's possible that all kids may not get their vaccines on schedule, leaving them vulnerable to diseases like whooping cough. On Tuesday, researchers at the National Immunization Program at the Centers for Disease Control and Prevention and Emory University reported that, indeed, vaccines are not being given on time.

What the researchers wanted to know: How often and extensive are delays in vaccination for children ages 2 and under?

What they did: In 2003, the CDC randomly surveyed households by phone to gather information on the vaccination schedules of 14,810 kids between the ages of 2 and 3. They asked when children received the recommended vaccines, then followed up with the kids' doctors or immunization clinics to confirm the dates given. Researchers were interested in how many days kids went "undervaccinated" (i.e., they weren't immunized according to schedule) for six vaccines: diphtheria/tetanus/whooping cough, polio, measles/mumps/rubella, flu, hepatitis B, and chicken pox. They also wanted to know what factors led to undervaccination.

What they found: Most kids got at least some shots late; only 26 percent received all their immunizations on time. About a third were undervaccinated for less than one month and 29 percent for between one and six months, while 37 percent had gaps in vaccinations for more than six months. And about 21 percent of kids in the survey were undervaccinated for more than six months and for four or more specific vaccines. Delays varied by vaccine, too; only 9 percent of the children were undervaccinated against polio for more than six months, while more than a fifth had at least a six-month delay in the recommended flu vaccine schedule. Researchers also discovered that kids whose vaccines were severely delayed were more likely to have a mother who wasn't married or didn't have a college degree, to live in a household with two or more kids, to be black, to have used multiple immunization providers, and to have used public clinics.

What it means to you: While the number of kids who have had all their vaccines is higher than ever before in this country, some children are made vulnerable by delays. Kids who are undervaccinated are at risk of getting sick, and they may also help fuel an outbreak of a disease that spreads easily (like measles) or is already present in the population (like whooping cough). Researchers pointed out, for example, that 14 percent of kids under 2 are undervaccinated for measles for at least three months. Parents can do their part to protect their own kids and other people by following the vaccination schedule, and the medical community needs to figure out how to reach the kids who are most likely to have their immunizations delayed.

Caveats: The survey was conducted by telephone, and it required the parent to accurately recall their child's vaccination schedule and for the provider to have kept accurate records. If those providers weren't identified or had incomplete information, kids may have been classified as undervaccinated when they actually did get their shots. There were also vaccine shortages in 2000 to 2002 that may have kept kids from getting some vaccines on time, though other research has shown the shortages didn't have a huge impact.

Find out more: The CDC has a recommended vaccine schedule for kids 5 and under, as well as information addressing parents' safety concerns.

Read the article: Luman, E.T. et al. "Timeliness of Childhood Vaccinations in the United States." The Journal of the American Medical Association. March 9, 2005.

Abstract online: http://jama.ama-assn.org