A new study says fear or complacency about vaccinations can allow preventable diseases such as polio, measles and whooping cough to spread. Scientists say preventing vaccine scares should be part of global immunization policies.

A virus may be very small, but it can travel very far, very fast. Professor Chris Bauch said fear of vaccinations can help send the virus on its way.

“Vaccine scares are quite old. For example, when the first vaccine ever invented, the smallpox vaccine, was introduced, there was a lot of resistance to that vaccine as well. And people were afraid that it would turn us into cow-like hybrids because of the connection between cowpox and smallpox. And it’s happened since then in many situations,” he said.

There was a scare in the 1970’s for a pertussis or whooping cough vaccine, which lowered immunization coverage in many countries. In the 1990s, there was a big scare over the measles, mumps and rubella vaccine after some claimed it caused autism. That allegation continues to this day.

Fear over the polio vaccine in parts of Nigeria had far-reaching repercussions.

“Polio had essentially been cornered to northern Nigeria. And when the vaccine scare happened for the polio vaccine the vaccine coverage dropped so much that polio resurged and it spread not only to other parts of Nigeria, but other countries as well. So that set back the polio eradication efforts by many years,” he said.

Overall, the stories received a lot of media attention.

Bauch is an associate professor of mathematics at the University of Guelph in Ontario, Canada. He and his colleagues developed a model looking at the relationship between disease and reaction to vaccinations.

“If you ask people why aren’t you vaccinating, one reason they’ll give is ‘I’m afraid of the vaccine.’ Another reason they might give is that ‘other parents have already been vaccinated so I don’t need to.‘ So some people who are not vaccinating are doing it because they know that they can rely upon other people having vaccinated already,” he said.

And if other children in the community are vaccinated, he said, some parents may believe their children won’t be infected. Another reason is pressure from other parents not to vaccinate.

“There seem to be two critical periods: First, when the vaccine is first introduced, because it’s a new product and many people have not been vaccinated with it yet, and so there may be distrust of it. And the second danger zone is perhaps years down the road when the disease is successfully controlled and there’s no more disease around. And so the memory of that fades and then people may start to free ride or may become afraid of the vaccine,” he said.

Bauch said when parents in developing countries see the effects of disease first hand they’re much more likely to have their children vaccinated. He says a successful immunization campaign needs a two pronged approach.

“First of all, you have to explain the data and explain the risks. And show them that here’s the vaccine risk and here’s the disease risk. And even if the disease is rare you’re still more likely to get seriously ill from the disease than you are from the vaccine,” he said, adding, “The second approach which is very effective is actually to talk about what you’ve done. For example, I have two children who are three and five and they’ve gotten all their vaccines. So obviously if I’m willing to vaccinate my own children then I must be telling the truth when I say it’s a safe product because I wouldn’t give something to my children that’s harmful.”

Bauch said the goal is to better understand how people behave and then give that information to policymakers.

“For example, we want to be able to tell the policymakers if a given country is perhaps more susceptible to vaccine scares than others – especially if it’s a new vaccine that’s being introduced and we don’t have history to go on. And second of all, we want to be able to tell them if a vaccine scare has happened and vaccine coverage has dropped to dangerously low levels, what are the best ways to get that up,” he said.

He warned that vaccine scares could actually become more common as disease “eradication goals are approached.” With fewer outbreaks, some may no longer see the need for vaccinations. Bauch said vaccines, at times, are victims of their own success. The study findings were published in PloS Computational Biology.