Seasonality of Pneumococcal Nasopharyngeal Carriage in Rural Gambia Determined within the Context of a Cluster Randomized Pneumococcal Vaccine Trial

BACKGROUND: We conducted an ancillary study among individuals who had participated in a PCV-7 trial in rural Gambia, to determine the influence of season on the prevalence of pneumococcal carriage. METHODS: 636 individuals above 30 months of age were followed from 4 to 20 months after vaccination with PCV-7 or meningococcal-conjugate-vaccine. Nasopharyngeal swabs were collected periodically between November 2006 and June 2008. Overall, 4,495 NPS were collected. RESULTS: Prevalence of pneumococcal nasopharyngeal carriage in the study subjects (median age 11 years) was 55.0%; this prevalence decreased linearly with increasing age (p = 0.001). Prevalence of carriage was significantly higher during the dry than the rainy season for any pneumococcal carriage 57.6% versus 47.8% (p<0.001), pneumococcal vaccine serotype carriage 10.3% versus 6.5% (p< 0.001) and non-vaccine serotype carriage 49.7% versus 42.7% (p<0.001). Differences remained significant in the adjusted analysis. CONCLUSIONS: In areas of Africa with marked variation in rainfall, seasonality of pneumococcal carriage needs to be considered when interpreting carriage data.