Table 3.

Illustration of the value of sewage surveillance alongside individual case testing for early warning and support of public health decisions.

Scenario	Surveillance		Interpretation	Possible Action
	Reported cases from individual testing	Sewage testing (e.g. weekly testing program)
Large city, following a COVID-19 wave.	Low number of daily reported cases	Low numbers in sewage (or not detected)	Consistent	•No change
		An increase in load by around an order of magnitude	Inconsistent •Could be early warning of an increase in cases. •Initiate follow-up sample	•Increase level of alert regarding increase in cases •Prepare health system for increase in cases presenting for testing/treatment •Implement public health mitigation strategies
Large city, in the midst of a COVID-19 wave	High numbers (increasing or plateauing) of daily reported cases	Load increasing or plateauing	•Cases still increasing •Public health interventions (as yet) ineffective	•Take action to improve compliance with public health measures •Increase stringency of public health measures
		Load decreasing	•The number of shedders and/or magnitude of shedding is decreasing. •Public health interventions are effective.	•No change
Small community, believed to be free of COVID-19	Zero reported cases	Not detected	Consistent	•Continue surveillance programs
		Detected	Inconsistent •One or more shedders present in (or visiting) the population. Use quantitative relationship to interpret the significance of the magnitude of concentration	•Further investigation •Increase level of alert regarding potential of cases in the community •Prepare health system for increased local testing and potential treatment of new cases.