While more than 90 percent of Oregon children are vaccinated against pertussis, this protection is not enough to stop pertussis from spreading when those choosing not to vaccinate share social networks, schools or other connections, according to a new study by the Oregon Immunization Program.

Boy with whooping cough/Video Screen Shot

“The Timing of Pertussis Cases in Unvaccinated Children in an Outbreak Year: Oregon 2012,” published in the current edition of The Journal of Pediatrics, suggests that relaxing immunization recommendations and school requirements could cause vaccine-preventable diseases to spread.

For the study, epidemiologists Steve Robison and Juventila Liko looked at a 2012 pertussis outbreak in northwest Oregon. They found that pertussis cases among unvaccinated children were three times more likely to occur earlier in an outbreak than cases among vaccinated children. This indicates that unvaccinated children were more frequently the source of pertussis disease which was then passed on to other children.

As the outbreak spread, the earliest cases in new disease-affected areas were also mostly among the unvaccinated. It is possible, the study found, that the unvaccinated have a greater role in how outbreaks develop beyond simply not helping build community protection.

“Parents who are reluctant to immunize their children often have social networks and other connections to others with similar lack of immunizations,” Robison said. “This may provide a way for infectious disease to spread across communities.”