Seasonality of tuberculosis in India: Is it real and what does it tell us?

India has a third of the world's tuberculosis cases. Large-scale expansion of a national programme in 1998 has allowed for population-based analyses of data from tuberculosis registries. We assessed seasonal trends using quarterly reports from districts with stable tuberculosis control programmes (population 115 million). In northern India, tuberculosis diagnoses peaked between April and June, and reached a nadir between October and December, whereas no seasonality was reported in the south. Overall, rates of new smear-positive tuberculosis cases were 57 per 100 000 population in peak seasons versus 46 per 100 000 in trough seasons. General health-seeking behaviour artifact was ruled out. Seasonality was highest in paediatric cases, suggesting variation in recent transmission.