On the Role of Different Age Groups and Pertussis Vaccines During the 2012 Outbreak in Wisconsin

There is limited information on the roles of different age groups in propagating pertussis outbreaks, and on the impact of vaccination on pertussis transmission in the community.

The relative roles of different age groups in propagating the 2012 pertussis outbreak in Wisconsin were evaluated using the relative risk (RR) statistic that measures the change in the group's proportion among all detected cases before vs after the epidemic peak. The impact of vaccination in different age groups against infection (that is potentially different from the protective effect against detectable disease) was evaluated using the odds ratios (ORs), within each age group, for being vaccinated vs undervaccinated before vs after the outbreak's peak.

The RR statistic suggests that children aged 13-14 years played the largest relative role during the outbreak's ascent (with estimates consistent across the 3 regions in Wisconsin that were studied), followed by children aged 7-8, 9-10, and 11-12 years. Young children and older teenagers and adults played more limited relative roles during the outbreak. Results of the vaccination status analysis for the fifth dose of DTaP (for children aged 7-8 years: OR, 0.44; 95% confidence interval [CI], 0.23-0.86; for children aged 9-10 years: OR, 0.51; 95% CI, 0.27-0.95); and for Tdap for children aged 13-14 years (OR, 0.38, 95% CI, 0.16-0.89) are consistent with protective effect against infection.

While our epidemiological findings for the fifth dose of DTaP and for Tdap are consistent with protective effect against infection, further studies, including those estimating vaccine effectiveness against infection/transmission to others particularly for pertussis vaccines for adolescents, are needed to evaluate the impact of vaccination on the spread of pertussis in the community.