Seasonality of tuberculosis in New York City, 1990-2007.

SETTING

Several non-US-based studies have found seasonal fluctuations in the incidence of tuberculosis (TB).

OBJECTIVE

The current study examined patterns of TB seasonality for New York City verified TB cases from January 1990 to December 2007.

DESIGN

Autocorrelation functions and Fourier analysis were used to detect a cyclical pattern in monthly incidence rates. Analysis of variance was used to compare seasonal mean case proportions.

RESULTS

A cyclical pattern was detected every 12 months. Of the 34,004 TB cases included, 21.9% were in the fall (September-November), 24.7% in winter (December-February), 27.3% in spring (March-May), and 26.1% in the summer (June-August). The proportion of cases was lowest in fall (P < 0.0001) and highest in the spring (P < 0.0002).

CONCLUSION

Possible explanations for seasonal variations in TB incidence include lower vitamin D levels in winter, leading to immune suppression and subsequent reactivation of latent TB; indoor winter crowding, increasing the likelihood of TB transmission; and providers attributing TB symptoms to other respiratory illnesses in winter, resulting in a delay in TB diagnosis until spring. Understanding TB seasonality may help TB programs better plan and allocate resources for TB control activities.