DENVER, Jan. 21, 2013 /PRNewswire/ -- In a new study published today in JAMA Pediatrics (formerly Archives of Pediatrics and Adolescent Medicine), Kaiser Permanente researchers found that 49 percent of children ages 2-24 months did not receive all recommended vaccinations, or did not get vaccinated according to the Advisory Committee on Immunization Practices schedule. Kaiser Permanente researchers used the Vaccine Safety Datalink — a collaborative effort among the Centers for Disease Control and Prevention and nine managed care organizations — to analyze immunization records of 323,247 children born between 2004-2008. Data from immunization records helped illuminate the number of days each child was missing scheduled vaccines for any reason, including parents intentionally choosing not to vaccinate their children according to ACIP recommendations. Children who did not receive their vaccines on time were considered "undervaccinated." "While a large majority of parents in the U.S. choose to vaccinate their children, a growing number of parents are concerned about vaccine safety and choose to vaccinate their children according to alternative immunization schedules," said lead study author Jason Glanz, PhD, a senior scientist at Kaiser Permanente's Institute for Health Research. "The medical community doesn't have a lot of data on these alternative schedules, so we are hoping the results from this study will open the door to more opportunities to examine their safety and efficacy." The study found the number of undervaccinated children increased significantly during the study period, and 1 in 8 undervaccinated children's parents intentionally chose not to adhere to ACIP immunization guidelines. Alternative schedules either involve increasing the time between vaccinations or reducing the number of vaccinations in a single office visit, leaving children undervaccinated. Study findings also indicate undervaccinated children are less likely to visit their doctor's offices and more likely to be admitted to hospitals, compared to their peers vaccinated under the standard schedule.