Viral gastroenteritis caused by Caliciviruses is found all over the world. This study aims to estimate the number of people infected with viral gastroenteritis based on the virus concentration in sewage collected from sewage treatment plants in 11 major cities in Hokkaido. Norovirus GI and GII were detected in all plants (11/11, 100%) and Sapovirus in 9 plants (9/11, 82%), with concentrations of Norovirus GI, GII, and Sapovirus ranging from 536 to 78,087 copies/L, 99,586 to 5,476,257 copies/L, and 0 to 18,767,409 copies/L, respectively. We constructed a formula to estimate the number of people infected with viral gastroenteritis based on the virus concentration in sewage, the processing capacity of sewage treatment plants, the fecal volume and viral concentration of infected people. Evaluated by this formula, the estimated number of infected people ranged from 367 to 33,563. The number of cases reported per sentinel is affected by the size of the population treated by the sewage treatment plant and lacks accuracy, and a discrepancy was confirmed from the estimated number of infected people. This study demonstrated that by using the developed calculation formula, it is possible to estimate the number of people infected with viral gastroenteritis based on the virus concentration in sewage, and that it is useful in understanding the actual situation of the epidemic.
