A whooping cough epidemic in 2012 in Washington state did not significantly change statewide vaccination rates, according to a study by the University of Washington School of Public Health.

The study, published online August 18 in Pediatrics, found no significant change in the proportion of infants who were up-to-date on their pertussis shots before and after the statewide pertussis epidemic of 2011 to 2012. Nearly a third of infants were not up-to-date with their shots, researchers found.

"This may challenge the conventional wisdom," said senior author Dr. Ali Rowhani-Rahbar, assistant professor of epidemiology. "Usually, we look to see if a drop in immunization results in an outbreak. But we were interested in looking at the relationship the other way around: Does an outbreak lead to increased immunization? We found that it did not in Washington state."

Dr. Rowhani-Rahbar said many factors affect parental decisions on whether to delay or refuse vaccines, including fears of adverse effects. But the added risk of adverse events following immunization is very small – roughly one in a few thousand for some of the most common events (e.g., febrile seizures following immunization with measles-containing vaccines), he said. Washington has one of the highest U.S. non-medical exemption rates in the country from required kindergarten vaccinations.

While overall immunization rates remained the same for the state, researchers found significant changes on the county level. Vaccination rates went up in urban King and Pierce Counties, while they dropped in three other populous counties: Spokane, Clark and Yakima. One possible reason, researchers said, was the robust media response to the epidemic in the western part of the state where the majority of whooping cough cases occurred. The research was led by Dr. Elizabeth Wolf as part of her master's thesis in epidemiology for the School of Public Health. Dr. Wolf is a senior fellow and acting instructor in pediatrics at Seattle Children’s Research Institute. Other research partners were the UW Department of Pediatrics, the Washington State Department of Health and the Washington State Immunization Information System.