Every year, parents are required to make sure their children are up-to-date on all of the necessary vaccinations – and some children are not allowed back to school unless they have not been properly immunized.

However, a new trend on the rise has many parents deviating from the recommend vaccine schedule – or simply disregarding it altogether.

According to a new study from the Kaiser Permanente Colorado Institute for Health Research, 49 percent of children between the ages of 2 and 24 do not receive all of the recommended vaccinations or do not get vaccinated at all.

The current recommended immunization schedule – which includes vaccines for hepatitis B, MMR (measles, mumps, and rubella) and more – was set in place by the Advisory Committee for Immunization Practices (ACIP) and details the ages at which children should receive specific shots. Some vaccines are often grouped together and administered all at once during one doctor’s visit.

Many medical experts argue that when it comes to vaccination effectiveness, there needs to be a level of “herd immunity” – the idea that contagious diseases can be disrupted or almost eradicated if large portions of the population are immune, stopping infections from spreading.

But, many parents are fearful that too many vaccines given at one time could overwhelm the child’s immune system, or the ingredients of the vaccines could produce long-term side effects.

“There are a growing number of parents who are expressing concerns about vaccines,” Jason Glanz, a senior scientist at Kaiser Permanente and lead author of the study, told FoxNews.com. “Some feel children receive too many, too soon or there are too many vaccines in too short of time, so they request alternative schedules. Despite these parents’ concerns, we don’t know if these alternative schedules are safer or less safe.”

Utilizing data from the Vaccine Safety Datalink – a collaborative effort between the Centers for Disease Control and Prevention (CDC) and nine managed care organizations (MCOs), Glanz and his colleagues analyzed records from 323,247 children born between 2004 and 2008. The researchers assessed each child’s immunization status, defining children as “undervaccinated” if they had not received vaccinations on time.

“We calculated what we call average days undervaccinated,” Glanz said. “…That was our primary metric. We looked at prevalence of that over time, is it increasing, what’s the magnitude of impact, etc. We attempted to look at other patterns, such as, we also compared the health care seeking behaviors of the parents of these two sets of children – those vaccinated on time and those not on time.”

Not only did nearly half of the children deviate from the recommend vaccine scheduled, but the amount of undervaccinated children increased from 2004 to 2008. The researchers found many parents increased time between vaccination visits or reduced the amount of vaccinations administered during a single visit. Also, the statistics could be more intentional than due to ignorance, as one in eight undervaccinated children had parents who intentionally chose to deviate from the ACIP recommendations.

Glanz said many parents – apart from fearing the safety of vaccines – are starting to believe that what the ACIP recommends is not what is right for their child.

“Some parents feel that they can better protect their children than through vaccination,” Glanz said. “Some parents who don’t vaccinate on time utilize alternative health care options.”

The new trends raise some concerns as to whether these undervaccinated children are at great risk for disease. The study did find that undervaccinated children were less likely to visit the doctor and more likely be admitted to hospitals than their peers who had adhered to the recommend vaccine schedule.

“It’s reasonable to assume that if you’re significantly undervaccinated, you’re at risk for vaccine preventable disease,” Glanz said. “The decision to not vaccinate puts them at risk for pertussis, chicken pox and more. Also vaccine preventable diseases cluster in regions where there are higher rates of vaccine refusal.”

However, Glanz said the purpose of this study is not meant to antagonize parents who do not strictly adhere to the recommend vaccination guidelines. The researchers hope to use their findings as more of a jumping off point for further research into the potential dangers – or even benefits – of changing the vaccination schedules.

“Parents concerns are legitimate, because we don’t see these diseases anymore, so we want to focus on safety of vaccines,” Glanz said. “… Parents just want to do what’s best for the kids, and that’s what we want too, so it’s really our common ground. It’s about having a conversation and listening to parents’ concerns, so we can help them make a better, more informed decision.”

The study was published Jan. 21 in JAMA Pediatrics.