  Individuals infected with SARS-CoV-2, the virus that causes COVID-19, may
shed the virus in stool before developing symptoms, suggesting that
measurements of SARS-CoV-2 concentrations in wastewater could be a "leading
indicator" of COVID-19 disease activity. Multiple studies have corroborated the
leading indicator concept by showing that the correlation between wastewater
measurements and COVID-19 case counts is maximized when case counts are lagged.
However, wastewater-based epidemiology for COVID-19 encompasses many possible
applications, and the meaning and quantification of a "leading indicator" may
be different for different applications. Here we enumerate three different
applications of wastewater-based epidemiology for COVID-19: a qualitative
"early warning" system; an independent, quantitative estimate of disease
activity; and a quantitative alert of bursts of disease activity. We find that
the leading indicator concept has different implications and utility for each
application and that wastewater's lead time will vary depending on
epidemiological, biological, and health systems factors. There is no one single
lead time for wastewater-based COVID-19 monitoring, and, at least in the case
of using wastewater to inform estimates of population-wide disease activity,
wastewater's lead time is not important to its public health utility.
