About half of infants and young children in the United States receive vaccinations late or not at all, a new study suggests.

Between 2004 and 2008, about 49 percent of children ages 2 months to 2 years were “undervaccinated,” meaning they received at least one vaccine or more a month later than is recommended by the current vaccine schedule. The percentage of undervaccinated children increased during the study period.

In about 13 percent of cases analyzed, parents intentionally delayed their child's vaccination, the researchers said.

Some parents say they intentionally delay vaccinations, because they are afraid their child is getting too many vaccines in too short a period, said study researcher Jason Glanz, a senior scientist at Kaiser Permanente's Institute for Health Research. However, research does not support such concerns — an Institute of Medicine report released last week found children who receive their vaccines on time are not at increased risk for medical conditions, such as developmental disorders.

Because of the many different ways a child could be considered undervaccinated, the exact effect of undervaccination on a child's health is not known. But experts say when parents deviate from the recommended vaccination schedule for their kids, they are taking unnecessary risks.

“When you make up a schedule, it is by definition an untested schedule, both in safety and effectiveness,” said Dr. Paul Offit, chief of the Division of Infectious Diseases at the Children's Hospital of Philadelphia, who was not involved in the study. Moreover, a delay in vaccination would prolong the period of time a child would be susceptible to a vaccine-preventable disease, Offit said. [See 5 Dangerous Vaccine Myths.]

A higher number of unvaccinated children also decreases so-called herd immunity — disease protection that occurs because a certain portion of people are vaccinated — and increases the risk of outbreaks of infectious disease, such as measles and whooping cough, Offit said.

Glanz and colleagues analyzed records from more than 323,200 U.S. children born between 2004 and 2008, and determined whether they received vaccinations on time, or 30 or more days late. Eight vaccines given over the first two years of life were included in the study.

During the study period, the percentage of undervaccinated children increased from 41.8 percent in 2004 to 54.4 percent in 2008, and was 49 percent for the period overall.

Besides parental choice, other reasons for undervaccination include missing doctors' visits, losing health-care coverage, or waiting to give a shot because a child is ill at the time he or she should receive it, Glanz said.

Considering whether each vaccine was received, delayed or never given, there were about 1,400 distinct ways in which children were undervacciated. The researchers hope the findings will be a first step in studying the safety of alternative vaccination schedules, Glanz said.

Previous studies have found children of parents who refuse vaccines are nine times more likely to get chickenpox, and 23 times more likely to get whooping cough than children who receive vaccines on time, the researchers said.

The new study is published online today (Jan. 21) in the journal JAMA Pediatrics.

Pass it on: Many young children don't receive recommended vaccines on time.

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