Multiple teams have also confirmed that sewage can serve as an early warning system: Waste water viral levels often peaked days before doctors saw a peak in official COVID-19 cases. This lead time, which can range from a couple of days to two weeks, depends partly on the robustness of local clinical testing programs, scientists say. When more people are being tested for the virus more frequently, the waste water data provides less advance warning. The lead time also exists because infected people often begin shedding the virus, SARS-CoV-2, before they feel symptoms and then, once they fall ill, frequently delay seeking medical care. “I think waste water has proven itself as one of the most, I would say, objective means of understanding what SARS-CoV-2 is doing in our society,” said Gertjan Medema, a microbiologist at KWR Water Research Institute in the Netherlands. Waste water surveillance is not a replacement for clinical testing, experts said, but can be an efficient, cost-effective complement. In one study published in August, researchers calculated that they could test the waste water from every treatment plant in Germany millions of times for less than it would cost them to test every German resident just once. The approach is likely to be especially valuable in low- and middle-income countries, where testing resources are even more limited.