Seasonal drivers of tuberculosis: evidence from over 100 years of notifications in Cape Town

BACKGROUND: Tuberculosis incidence varies seasonally in many settings. However, the role of seasonal variation in reactivation vs. transmission is unclear. METHODS : We reviewed data on TB notifications in Cape Town, South Africa, from 1903 to 2017 (exclusive of 1995-2002, which were unavailable). Data from 2003 onward were stratified by HIV status, age and notification status (new vs. retreatment). We performed seasonal decomposition and time-dependent spectral analysis using wavelets to assess periodicity over time. We estimated monthly peak-to-peak seasonal amplitude of notifications as a percentage of the annual notification rate. RESULTS : A seasonal trend was intermittently detected between 1904 and 1994, particularly during periods of high notification rates, but was consistently and strongly evident between 2003 and 2017, with peaks in September through November, following winter. Among young children, a second, higher seasonal peak was observed in March. Seasonal variation was greater in children (< 5 years, 54%, 95% CI 47-61; 5-14 years, 63%, 95% CI 58-69) than in adults (36%, 95% CI 33-39). CONCLUSIONS : Stronger seasonal effects were seen in children, in whom progression following recent infection is known to be the predominant driver of disease. These findings may support increased transmission in the winter as an important driver of TB in Cape Town.