With so much conflicting information readily available to parents, Dr. Dina Pfeifer, program manager for vaccine-preventable diseases and immunization for WHO's Europe office, said she believes the decision of whether to immunize children has become so fraught that many parents choose to do nothing at all.
"They have a difficulty dealing with the amount of information for and against (vaccination) on the Internet, and out of this confusion they are failing to recognize the risks of the disease," she said.
Another factor driving parents' decision not to vaccinate is the security that comes with herd vaccination, as rates of immunization for many diseases remain above 92% for the population.
But Europe's recent battle with measles demonstrates the problems under-vaccinated populations can pose, especially with older children.
"Europe had 100,000 cases of measles from 2009 to 2012, and that shows how prevalent the pockets of un-immunized populations are in that area," Pfeifer said. "Almost 50% of those cases were older than 10 years of age, and the older you are when you contract measles, the more severe the course of the disease."
Another factor of these pockets is their affluence; these parents tend to be the ones able to afford overseas travel.
In 2008, a 7-year-old U.S. boy whose parents chose not to immunize him against measles traveled with his family to Switzerland. He caught the virus and returned to San Diego, unknowingly exposing 839 people to the disease and infecting 11 unvaccinated children, according to the journal Pediatrics.
In Europe and the United States, parents and most people under 45 have never seen the effects of diseases such as measles, diphtheria or polio.
"The fear factor (among parents in the developed world) is missing now -- the knowledge of what's on the other side if you don't have vaccinations," said Dr. Seth Berkley, CEO of the Global Alliance for Vaccines and Immunization, known as GAVI Alliance.
The lack of knowledge of these diseases is also a problem among younger doctors and pediatricians, who may not be able to identify the signs, resulting in misdiagnoses.
"There is a lot of value in case-based learning, but it is difficult to learn how to recognize these diseases if you haven't seen them before," said WHO's Pfeifer.
In contrast, most parents in the developing world, in places such as Somalia, have seen family members suffer, be maimed or die from such diseases, health advocates said.
Education and motivation
To address the problem, Berkley prescribes localized programs in countries to supplement the already high overall levels of immunization. Other physicians are supporters of parental education and want to ensure parents feel free to ask as many questions of their doctors and health care workers.
Dr. Steve Hambleton of the Australian Medical Association said further motivation may be necessary. "When you incentivize the parents in a meaningful way, whether it be financial or with other incentives, you can make an enormous difference in vaccination uptake," he said.
Berkley, a doctor who specializes in epidemiology and global health, said he has seen the devastating effects of vaccine-preventable diseases in war-torn countries and refugee camps.
Berkley said he wished he could take some parents in the developed world "on a tour, show them how horrible it is. Show them the illness that occurs out of these viruses."
"We've brought down child mortality dramatically with these vaccination campaigns and we are making dramatic progress, but the challenge is getting people to understand what the world was like before this."