Original ArticlesThe Timing of Pertussis Cases in Unvaccinated Children in an Outbreak Year: Oregon 2012

ObjectiveTo assess whether, during a 2012 pertussis outbreak, unvaccinated and poorly vaccinated cases occurred earlier on a community level.Study designPediatric pertussis among children 2 months to 10 years of age in the Oregon Sentinel Surveillance region during an epidemic starting at the beginning of 2012 were stratified by immunization status, age, zip code, and calendar date of disease onset. Differences in median onset as days between fully or mostly vaccinated, poorly vaccinated, and unvaccinated cases were examined overall and within local zip code areas. Disease clusters also were examined using SatScan analysis.ResultsOverall, 351 pertussis cases occurred among children aged 2 months to 10 years of age residing in 72 distinct zipcodes. Among unvaccinated or poorly vaccinated cases, their median date of onset was at calendar day 117 (April 26, 2012), whereas for those who were fully or mostly vaccinated the median date of onset was 41 days later, at day 158 (June 6, 2012). Within each local zip code area, the unvaccinated cases were 3.2 times more likely than vaccinated cases to have earlier median dates of onset (95% CI 2.9-3.6).ConclusionIn this outbreak, pertussis cases among unvaccinated children represented an earlier spread of disease across local areas. Controlling outbreaks may require attention to the composition and location of the unvaccinated.