More and more babies and toddlers aren’t getting their recommended vaccines on time, a new study suggests.

Of more than 300,000 U.S. kids born between 2004 and 2008, almost half were “undervaccinated” at some point before their second birthday - in some cases because parents chose to forgo shots recommended by the Centers for Disease Control and Prevention.

Researchers said that trend is cause for concern because if enough kids skip their vaccines, whole schools or communities may be at higher risk for preventable infections such as whooping cough and measles.

“When that happens, it can create this critical mass of susceptible individuals,” said Saad Omer, from the Emory Vaccine Center in Atlanta.

“For some vaccinated kids, their risk of getting the disease also goes up,” Omer, who wasn’t involved in the new study, told Reuters Health.

That’s because no vaccine protects recipients perfectly from infection. So public health officials rely, in addition, on so-called “herd immunity” to keep vaccine-preventable diseases from spreading.

For their report, Jason Glanz from Kaiser Permanente Colorado in Denver and his colleagues analyzed data from eight managed care organizations, including immunization records for about 323,000 kids.

During the study period, the number of kids 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 toward the end of the study were late on their vaccines for more days, on average, than those born earlier.

Just over one in eight kids went undervaccinated due to parents’ choices. For the rest, it wasn’t clear why they were late with 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.

Undervaccinated kids also tended to have fewer doctors’ appointments and emergency room visits than those who got their shots on time, according to findings published Monday in JAMA Pediatrics.

That could be because their parents more often turn to alternative or complementary medicine when it’s an option, Omer said.

Recent studies have shown many parents are asking to delay or skip certain vaccines, often citing safety concerns such as a link between vaccines and autism - a theory which scientists now agree holds no water (see Reuters Health story of Nov 28, 2011 here: reut.rs/tvpjVf).

“We don’t really know if these ‘alternative schedules’ as they’re called are as safe, less safe or more safe than the current schedule,” Glanz told Reuters Health.

“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? It’s possible that some of these diseases that we worked so hard to eliminate come back,” he said.

Glanz said any parents who are considering an alternative vaccination schedule should talk with their child’s doctor first - and be especially careful about what they read online.

“We don’t have any evidence that there are any safety concerns with the current recommended schedule, and right now the best way to protect your child from infection is to get your child vaccinated on time,” he said.

SOURCE: JAMA Pediatrics, online January 21, 2013