Seasonal pattern of tuberculosis in Hong Kong

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Abstract
Background Summer predominance of tuberculosis (TB) was reported previously in temperate regions. No consistent data were available for lower latitudes. Method The monthly TB notification data in Hong Kong from 1991 to 2002 were examined for seasonal fluctuation. A seasonal model was then developed after standardization by period, sex, age, history of TB, form of disease, and bacteriological status. Results The raw monthly counts showed remarkably consistent seasonal fluctuation across different periods, sexes, and age groups. A sine model was fitted for 82 104 notifications (adjusted R2 = 0.373, P < 0.001). A summer peak was observed with seasonal fluctuation of 18.4% (P < 0.001), which was substantially higher than that reported previously for temperate regions. The amplitudes of fluctuation were 35.0, 15.0, 19.0, and 20.2% for those aged ≤14, 15–34, 35–64, and ≥65 years, respectively (all P < 0.001). No gender difference was noted (18.2% vs 19.0%, P = 0.790). Seasonal pattern was detected among new cases (18.6%, P < 0.001), but not retreatment cases (5.2%, P = 0.333). Culture-positive cases showed greater fluctuation than culture-negative cases (29.4% vs 6.4%, P < 0.001). No significant difference was found between pulmonary and extrapulmonary cases (16.8% vs 21.6%, P = 0.356). TB cases notified in summer were more likely to be smear-positive [odds ratio (OR) 1.100, 95% confidence interval (CI) 1.045–1.158, P < 0.001] and culture-positive (OR 1.175, 95% CI 1.121–1.232, P < 0.001) than those notified in winter, even after stratification by other key variables. Conclusion A consistent seasonal pattern was found, with variable amplitudes of fluctuation in different subgroups and differing disease characteristics in different seasons. These observations are suggestive of the presence of a seasonal disease-modifying factor.