Sewage monitoring uncovers ‘invisible’ COVID-19 cases missed by testing

As COVID-19 testing becomes less routine, official case numbers can make outbreaks look smaller than they really are. A research team led by Professor Michio Murakami has now shown that wastewater surveillance can uncover this “invisible” spread, providing a more objective picture of community infections and offering early warning signs for hospital-acquired cases. The study is published in the journal Environment International.

Traditionally, public health authorities have relied on data from clinical tests, such as PCR, to monitor infectious disease outbreaks. However, this approach has limitations when the number of tests performed decreases. For instance, after a change in public health policy or a drop in public concern, the official statistics may no longer reflect the true prevalence of the virus. Infections in asymptomatic individuals or those who do not seek testing go unrecorded, leading to a potential underestimation of the infection risk.

The research team compared viral concentrations in wastewater with the number of reported cases and clinical tests conducted in Sapporo, Japan. They found that after COVID-19 was reclassified to a lower alert level in Japan, the number of reported infections diverged significantly from the high viral loads still present in sewage. The study quantitatively confirmed that this disparity was directly attributable to the sharp decrease in the number of clinical tests being performed. This finding indicates that statistics based solely on clinical testing can be misleading.

This study underscores the critical importance of a multi-layered surveillance system that does not depend too heavily on clinical testing. Wastewater surveillance offers an objective tool for grasping the overall infection situation in a community. The findings can help strengthen the basis for public health decisions, such as adjusting alert levels, implementing infection control measures in health care facilities, and guiding public information campaigns. The research contributes to future public health policy by highlighting a robust method for monitoring infectious diseases, even when testing capacity is limited.

“A decrease in reported cases doesn’t always mean a lower infection risk,” says lead author Professor Murakami. “Our study uses independent wastewater data to show how a decline in testing can obscure the true scale of an outbreak. Wastewater surveillance offers an objective view of community-wide infection levels, unaffected by changes in testing policies or public behavior. It highlights the need for a multi-layered surveillance system.”