We investigated the relationship between epidemics and soil radiation through an exploratory study using sentinel surveillance data (individuals aged <20 years) during the last three epidemic seasons of influenza and norovirus in Japan.
We used a spatial analysis method of a geographical information system (GIS). We mapped the epidemic spreading patterns from sentinel incidence rates.
We calculated the average soil radiation [dm (μGy/h)] for each sentinel site using data on uranium, thorium, and potassium oxide in the soil and examined the incidence rate in units of 0·01 μGy/h.
The correlations between the incidence rate and the average soil radiation were assessed. Epidemic clusters of influenza and norovirus infections were observed in areas with relatively high radiation exposure.
A positive correlation was detected between the average incidence rate and radiation dose, at r = 0·61–0·84 (P < 0·01) for influenza infections and r = 0·61–0·72 (P < 0·01) for norovirus infections.
An increase in the incidence rate was found between areas with radiation exposure of 0 < dm < 0·01 and 0·15 ⩽ dm < 0·16, at 1·80 [95% confidence interval (CI) 1·47–2·12] times higher for influenza infection and 2·07 (95% CI 1·53–2·61) times higher for norovirus infection.
Our results suggest a potential association between decreased immunity and irradiation because of soil radiation.
Further studies on immunity in these epidemic-prone areas are desirable.