Children with parents in the military may have lower vaccination rates than other kids, according to a large U.S. survey.

Even with socioeconomic factors taken into account, parents’ memories and doctors’ records suggested that more military children under age three weren’t up-to-date on their childhood vaccinations: 28 percent, compared with about 21 percent of other kids, researchers reported in Pediatrics.

"Military children did not have documentation of full vaccination status as frequently as non-military," Dr. Angela Dunn, an epidemiologist for the Centers for Disease Control and Prevention, said by email. "We don't know if it's the result of the information that providers had access to or the actual vaccination status."

Dunn and colleagues used data from the National Immunization Survey (NIS), an annual phone interview that asks parents about their kids' vaccinations and then asks them to provide contact information for their pediatrician to verify which shots were administered.

The researchers analyzed NIS data from 2007 to 2012 for nearly 104,000 children nationwide, ages 19 to 35 months, who had complete information provided by parents and by health care providers. Of the total study population, 3,421, or 2.8 percent, were military dependents.

They looked at whether children were up-to-date on all recommended vaccines up to age three: at least four doses of a shot called DTaP (which protects against diphtheria, tetanus, and whooping cough/pertussis); three doses of the polio vaccine; one dose of a vaccine for measles, mumps and rubella; three doses of a shot called Hib that protects against bacterial infections including meningitis and pneumonia; three doses of the hepatitis B vaccine; and at least one shot for chicken pox.

Because of an Hib vaccine shortage from 2007 to 2009, the researchers excluded that one from the study.

Military dependents had lower levels of coverage for each of the recommended vaccines, but the differences were only statistically significant for polio and DTaP.

The researchers say that in addition to being military dependents, factors that made kids less likely to be up-to-date on vaccines included being younger than 30 months, having mothers without a college degree or unmarried parents, and moving from state to state or receiving vaccines from more than one health care provider.

Multiple moves are the most likely culprit for the lower vaccination rates found in the study, said Dr. Robert Frenck, a retired Navy pediatrician who currently practices at Cincinnati Children's Hospital Medical Center. Moving might disrupt or delay care, and it might also lead to incomplete medical records, Frenck, who wasn't involved in the study, said by email.

"If the family forgot about a move, or a health care provider, the study would not be able to capture the data," Frenck said. "Also, if the family has lost their immunization record, the new provider would not have a record."

Given the mobility inherent in military life, the lack of a single vaccine registry for all branches of the armed services might also be contributing to incomplete records, Dr. Dennis Conrad, a pediatrician at the University of Texas Health Science Center in San Antonio, said by email.

"The NIS database is a great tool for the U.S. as a whole; it may not be the right tool to drill down to what the military vaccination rates are," said Lt. Col. Amy Costello, head of immunization healthcare operations for the Defense Health Agency, Public Health Division. The survey has enough kids to be representative of the general population, but too few military children to reflect what's happening for that group, Costello said.

It should become easier to track vaccinations for military kids with the next generation of military electronic health records, designed to include more complete vaccine records and make it easier to monitor children who move as well as kids with two parents in different branches of the military, said Costello, who wasn't involved in the study.

To ensure children are well-protected against vaccine-preventable diseases, a national vaccination registry would be ideal, Dr. Douglas Diekema, a pediatrician at Seattle Children's Research Institute, said by email. "Without that, families will need to try to keep a copy of each child's vaccination records and provide those to their child's new medical clinic when they move to a new home."