Seasonal immune modulation in humans: observed patterns and potential environmental drivers

Cyclical fluctuations in host immunity have been proposed as a driver of respiratory infection seasonality, however few studies have attempted to directly assess whether or not seasonal immune modulation occurs in humans.

We reviewed studies assessing immune status at different times of the year, restricting our review to studies assessing any of the following three biomarkers: antibody responses following vaccination, delayed-type hypersensitivity responses following skin testing, and clinical responses following experimental infection.

After systematic review and critical appraisal of the literature, six separate studies were available for final discussion. These results indicate that human immunity does vary by season. In the tropical setting of West Africa, both cell mediated and humoral immune responses appear to be reduced in children during the rainy season. In the tropical setting of Bangladesh, cell mediated immune responses also appear to be reduced in children during the rainy season. In the temperate setting of Russia, resistance to influenza infection appears to be reduced in young adults during winter.

Seasonal variation in immunity appears to occur in humans, and it is plausible that this variation may contribute to the seasonality of respiratory infections. Further research to assess the extent of seasonal immune modulation is required. We outline a number of recommendations to minimise bias in future studies.