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We wish to confirm previous reports of the seasonality of tuberculosis.1 2 The Second National Prevalence Study (SNPS) documented the prevalence of hospital and community acquired infections in 37 111 hospitalised patients in 157 centres in the UK and Ireland between 1993 and 1994.3 Hospital acquired lower respiratory tract infections were reported in 2.4% of the population (882 cases). Community acquired lower respiratory tract infections were reported in 6.1% of patients (2282 cases), of which 55 cases were tuberculosis. The prevalence rate of community acquired tuberculosis in the winter quarters (January to March, October to December) was 1.0/1000 patients and doubled in the summer months (April to June, July to September) to 1.9/1000 patients (OR 1.72, 95% CI for OR 1.0 to 2.97, p⩽0.05). The mechanism for such an increase in prevalence is unclear, although it is hypothesised that the seasonal fluctuation in vitamin D serum levels may contribute to impaired host defence mechanisms to Mycobacterium tuberculosis.1 4 We are unaware of any publication relating to the date of onset of symptoms as opposed to the date of diagnosis or notification. Are people more tolerant of coughs in the winter?
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