Examining the role of different age groups, and of vaccination during the 2012 Minnesota pertussis outbreak

There is limited information on the roles of different age groups during pertussis outbreaks. Little is known about vaccine effectiveness against pertussis infection (both clinically apparent and subclinical), which is different from effectiveness against reportable pertussis disease, with the former influencing the impact of vaccination on pertussis transmission in the community. For the 2012 pertussis outbreak in Minnesota, we estimated odds ratios for case counts in pairs of population groups before vs. after the epidemic's peak. We found children aged 11-12y, 13-14y and 8-10y experienced the greatest rates of depletion of susceptible individuals during the outbreak's ascent, with all ORs for each of those age groups vs. groups outside this age range significantly above 1, with the highest ORs for ages 11-12y. Receipt of the fifth dose of DTaP was associated with a decreased relative role during the outbreak's ascent compared to non-receipt [OR 0.16 (0.01, 0.84) for children aged 5, 0.13 (0.003, 0.82) for ages 8-10y, indicating a protective effect of DTaP against pertussis infection. No analogous effect of Tdap was detected. Our results suggest that children aged 8-14y played a key role in propagating this outbreak. The impact of immunization with Tdap on pertussis infection requires further investigation.