"What we're worried about is if (undervaccination) becomes more and more common, is it possible this places children at an increased risk of vaccine-preventable diseases?" said study leader Jason Glanz, with Kaiser Permanente Colorado in Denver.
"It's possible that some of these diseases that we worked so hard to eliminate (could) come back."
Glanz and his colleagues analyzed data from eight managed care organizations, including immunization records for about 323,000 children.
During the study period, the number of children who were late on at least one vaccine - including their measles, mumps and rubella (MMR) and diphtheria, tetanus and pertussis (DTaP) shots - rose from 42 percent to more than 54 percent.
Babies born towards the end of the study were late on their vaccines for more days, on average, than those born earlier.
"When that happens, it can create this critical mass of susceptible individuals," said Saad Omer, from the Emory Vaccine Center in Atlanta, who wasn't involved in the new study.
Just over one in eight children went undervaccinated due to parents' choices. For the rest, it wasn't clear why they were late getting their shots. Some could have bounced in and out of insurance coverage, Glanz suggested, or were sick during their well-child visits, so doctors postponed vaccines.
The report comes on the heels of new data from the Institute of Medicine saying that the recommended infant vaccine schedule is safe for children.
Image: Baby vaccine, via Shutterstock