(AP Photo/Eduardo Verdugo)

When an outbreak of a vaccine-preventable infectious disease hits, how might people in the affected area respond?

You might think once parents become aware of vaccination's benefits, they would choose to get their children fully immunized. As it turns out, at least in one case, that conventional wisdom didn't hold. Washington state's 2011-2012 whooping-cough outbreak didn't increase the number of infants who were fully up-to-date with their whooping cough immunizations, a new study suggests.

In recent years the U.S. has experienced a number of outbreaks of vaccine-preventable infectious diseases such as pertussis (whooping cough) and measles, both previously rare in the United States. Vaccination rates have actually grown nationally over the past couple of decades. But health officials in recent years have reported increases in the fraction of parents who don't have their children fully vaccinated or who seek vaccination exemptions for their children.

Often, large numbers of non-vaccinated or under-vaccinated children are clustered together, particularly in areas or states where it's easier for parents to get exemptions from school vaccination requirements. The rareness of such diseases might have reduced public awareness of the benefits of the vaccinations that are so crucial to preventing their spread. But it's also clear that the anti-vaccine movement has gained more traction.

And rising proportions of non-vaccinated or under-vaccinated children very likely played at least some role in past whooping-cough outbreaks. Research has linked clusters of non-medical exemptions in California to the state's whooping cough outbreaks in 2010.

With whooping cough, another factor may be at play. For reasons researchers still are trying to understand, a relatively new formulation of the children's whooping-cough vaccine doesn't appear to work as well as its predecessor. Although the vaccine still protects most people who get it, that protection may wane faster than before. That doesn't mean we should stop administering the vaccine; rather, it means that the public-health risks of not vaccinating become magnified.

But Washington state has had a strong anti-vaccine movement, and it shows in how many parents received non-medical vaccination exemptions for their kids in recent years. In the late 2000s, the percentage of kindergarteners who received at least one exemption from a vaccination surged, peaking at a whopping 7.6 percent in the 2008-2009 school year.

The state went on to suffer an epidemic of whooping cough in 2011 and 2012 that made national headlines. Since then, the state's recently toughened requirements to receive an exemption have helped cut exemption rates, but the damage may have already been done.

In the epidemic's immediate aftermath, did vaccination rates rise for whooping cough? To answer this question, Elizabeth Wolf of the University of Washington-affiliated Seattle Children's Research Institute and her colleagues assessed state immunization records from September 2011, before the epidemic, and September 2012, after it had started, and they determined whether Washington infants (3 to 8 months old) were up-to-date on their diphtheria, tetanus and pertussis (DTaP) vaccine. Infants were considered up-to-date if they had received one dose of the DTaP vaccine by 3 months old, another dose second by 5 months old and a third dose by 7 months old.

Wolf and colleagues had predicted that a lot more infants statewide would be up-to-date after the outbreak. But to their surprise, the proportion of infants who were up-to-date increased only 2.1 percentage points, from 67.4 to 69.5 percent. That change, the researchers said, was too small from a statistical standpoint for them to conclude that more infants were up-to-date post-epidemic. What do the findings tell us? "This finding may challenge the assumption that vaccine acceptance uniformly increases when risk of disease is high," Wolf said in a statement. She and her colleagues presented the findings at a recent scientific meeting in Vancouver, British Columbia.

It's only one study of one outbreak, so in that regard we should be cautious in how we apply these findings to other outbreaks. But the findings should give pause to public-health officials, who are already having to combat strong anti-vaccine sentiment as they try to encourage parents to keep their children up-to-date on their vaccinations. If an epidemic isn't enough to boost vaccination rates, one might wonder: What is?

"Our results support the idea that there is likely not one issue or strategy that will convince all hesitant parents," Wolf told me in an email. Researchers will have to figure out which combinations of approaches work best on specific groups of hesitant parents, she suggested.