Winter-seasonal epidemics of pneumococcal disease provide an opportunity to understand the drivers of incidence. We sought to determine whether seasonality of invasive pneumococcal disease is caused by increased nasopharyngeal transmission of the bacteria or increased susceptibility to invasive infections driven by co-circulating winter respiratory viruses.

METHODS. We analyzed pneumococcal carriage and invasive disease data collected from children <7 years in the Navajo/White Mountain Apache populations between 1996 and 2012. Regression models were used to quantify seasonal variations in carriage prevalence, carriage density, and disease incidence. We also fit a multivariate model to determine the contribution of carriage prevalence and respiratory syncytial virus (RSV) activity to pneumococcal disease incidence while controlling for shared seasonal factors.

RESULTS. The seasonal patterns of invasive pneumococcal disease epidemics varied significantly by clinical presentation: bacteremic pneumococcal pneumonia incidence peaked in late-winter while invasive non-pneumonia pneumococcal incidence peaked in autumn. Pneumococcal carriage prevalence and density also varied seasonally, with peak prevalence occurring in late-autumn. In a multivariate model, RSV activity was associated with significant increases in bacteremic pneumonia cases (attributable percent: 15.5%, 95%CI: 1.8-26.1%) but was not associated with invasive non-pneumonia (8.0%, 95%CI: -4.8-19.3%). In contrast, seasonal variations in carriage prevalence were associated with significant increases in invasive non-pneumonia infections (31.4%, 95%CI: 8.8-51.4) but not with bacteremic pneumonia.

CONCLUSIONS. The seasonality of invasive pneumococcal pneumonia could be due to increased susceptibility to invasive infection triggered by viral pathogens, while seasonality of other invasive pneumococcal infections might be primarily driven by increased nasopharyngeal transmission of the bacteria.