Even if people who are infected with the coronavirus never seek testing or treatment, they shed the virus in their stool. That has made wastewater surveillance a useful way to keep track of how much virus is circulating in a community. At the height of the pandemic, wastewater data provided an early warning of coming surges and helped experts monitor the spread of new variants.

Now that the emergency phase of the pandemic has ended, wastewater surveillance is an even more crucial tool, experts say. This spring, the Centers for Disease Control and Prevention stopped calculating community Covid levels and collecting some other types of tracking data.

Although hospitalization rates have fallen, the virus still poses dangers, especially to people with compromised immune systems. Local wastewater data can help people and institutions, such as long-term care facilities, make more informed decisions about when to take more precautions, the researchers said.

In September 2020, the C.D.C. established a National Wastewater Surveillance System to centralize and standardize local efforts to track the coronavirus in sewage. Since then, the system has expanded to cover more than 1,000 sampling sites across the country.

For the new study, the researchers analyzed the publicly available wastewater data for 268 counties participating in the national surveillance system. They compared the wastewater trends in each county to the local case and hospitalization rates during the first three quarters of 2022.

From January to March, a period that overlapped with the winter Omicron wave, high levels of virus in the wastewater closely mirrored high case and hospitalization rates, the scientists found. The wastewater levels were lower during the April-to-June time frame, before rising again in the July-to-September period. But during these final three months, when wastewater levels were high, official case and hospitalization rates remained relatively low.

“Even though we saw a bump in the case rates, it wasn’t as significant as what we saw in the wastewater data,” said Dr. Meri Varkila, a postdoctoral fellow at Stanford University and an author of the new study. “We figured that wastewater is probably the more accurate estimate of what’s actually happening in terms of Covid occurrence in communities nationwide.”

The coronavirus is constantly evolving. “We are assuming that inherently the virus hasn’t changed dramatically in the way that it sheds and in the way that it behaves in the environment,” said Dr. Shuchi Anand, a nephrologist at Stanford and an author of the study. But there may be future variants that show up in sewage either more or less readily than past variants have.

The population’s susceptibility will change, too, as people experience additional infections, new vaccines are rolled out, and immunity waxes and wanes. These shifts could make it difficult to predict what future spikes in wastewater levels mean for public health.

The wastewater data will remain most useful when viewed alongside other public health metrics, the researchers said.

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