Does vitamin D deficiency account for ethnic differences in tuberculosis seasonality in the UK?

Notifications of tuberculosis in England and Wales are reported to peak in the summer season. The purpose of this study was to confirm that finding and to determine to what extent patients of Indian Subcontinent (ISC) ethnic origin contributed to the seasonality. The clinical presentation of the disease is presumed to occur some months following reactivation of the endogenous latent focus of tuberculosis infection. There arises the possibility of vitamin D deficiency producing immunological inadequacy at the end of winter and beginning of spring.

Monthly (or 4-weekly) aggregated data over 7 years were collected from the three countries of mainland Britain, England, Wales, Scotland and from the city of Birmingham in England. The notifications from Birmingham were divided into those of ISC ethnic origin and 'whites'. The presence or absence of seasonality was determined by fitting a sinusoidal curve by the technique called 'cosinor analysis'. In this method amplitude gives a measure of the extent of the seasonal variation.

The summer peak of clinical diagnosis was confirmed in the UK series from England, Wales and Scotland. In England and Wales without Scotland a larger seasonal variation was present. Scotland, with a lower proportion of population of ISC ethnic origin, was examined separately and the results in Scotland alone failed to confirm seasonality. In the data from Birmingham, seasonality was confirmed with a greater amplitude, particularly in those over 60 years of age. The finding was influenced by those of ISC ethnic origin, seasonality not being present in the 'white' population.

The results from Birmingham are very striking, but there were almost three times as many patients in the ISC ethnic group as in indigenous 'white' patients. A series with larger numbers of 'white' patients would be necessary to confirm the absence of seasonality in the 'white' population. The discussion reviews the evidence that vitamin D may have an important hormonal role in immunological defence in the prevention of tuberculosis.