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

Abstract

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.

Temporally Varying Relative Risks for Infectious Diseases

“…[18]. The results highlight the important role played by children aged 8–14, particularly 11-12 year olds, in driving the 2012 outbreak in Minnesota.…”

“…Such groups experience a disproportionate depletion of susceptible members during the outbreak's early stages and represent a relatively smaller proportion of all cases of infection in the population during the outbreak's later stages. Figure 1 exhibits this phenomenon for children aged 10-14 during the 2010 pertussis epidemic in California (see also Figure 1 in [18] for the 2012 pertussis epidemic in Minnesota). Change in the relative risk of disease among different groups can be captured by the summary statistic RR (age-specific risk ratio) defined as the ratio of the proportion of a given age group g among all detected cases before the epidemic peak ( Bp ( g )) and the corresponding proportion after the epidemic peak ( Ap ( g )): $$RR\left(g\right)=\frac{Bp\left(g\right)}{Ap\left(g\right)}$$ Figure 2 illustrates the RR concept for a simulated epidemic in an age-stratified population.…”

“…Therefore, it is desirable to have statistical tools that make use of limited data, usually obtained from detected cases during infectious disease outbreaks, to make conclusions about the role of different population groups in propagating the spread of infection, the impact of vaccination, and other phenomena. Examples of such summary statistics that characterize temporal changes in the distribution of cases during epidemics were described by Worby et al [17,18]. We describe those methods below in the context of influenza and pertussis epidemics.…”

“…Importantly, while case detection (in this instance, hospitalization) rates may be highly age specific , they are not expected to vary over time within a particular age group. As a consequence, the ratio of the summary statistics RR for any pair of age groups g1 and g2 ( OR ( g1,g2 )= RR ( g1 )/RR( g2 )) is independent of the age-specific case detection rates [18], as long as these rates in each age group remain unchanged over the course of the epidemic, or all change by the same factor. This ratio can be used to statistically differentiate the relative prominence of different age groups during the outbreak's ascent (see e.g.…”

“…This ratio can be used to statistically differentiate the relative prominence of different age groups during the outbreak's ascent (see e.g. Table 2 in [18]).…”