Age-dependent association between SARS-CoV-2 cases reported by passive surveillance and viral load in wastewater

Omori, R.; Miura, F.; Kitajima, M.

The true number of individuals infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is difficult to estimate using a case-reporting system (i.e., passive surveillance) alone because of asymptomatic infection. While wastewater-based epidemiology has been implemented as an alternative/additional monitoring tool to reduce reporting bias, the relationship between passive and wastewater surveillance data has yet to be explicitly examined. Since there is strong age dependency in the symptomatic ratio of SARS-CoV-2 infections, this study aimed to estimate i) an age-dependent association between the number of reported cases and the viral load in wastewater and ii) the time lag between those time series. The viral load in wastewater was modeled as a combination of contributions from different age groups' virus shedding, incorporating the delay, and fitted with daily case count data collected from the Massachusetts Department of Public Health and SARS-CoV-2 RNA concentrations in wastewater collected by the Massachusetts Water Resources Authority. The estimated lag between the time series of viral loads in wastewater and of reported cases was 10.8 days (95% confidence interval =[10.2, 11.6]) for wastewater treatment plant's northern area and 8.8 days [8.4, 9.1] for southern area. The estimated contribution rate of a reported case to the viral load in wastewater in the 0-19 yr age group was 0.38 [0.35, 0.41] for northern area and 0.40 [0.37, 0.43] for southern area, that in the 80+ yr age group was 0.67 [0.65, 0.69] for northern area and 0.51 [0.49, 0.52] for southern area. The estimated lag between those time series suggested the predictability of reported cases ten days later using viral loads in wastewater. The contribution of a reported case in passive surveillance to the viral load in wastewater differed by age, suggesting a large variation in viral shedding kinetics among age groups.