Why It Matters: Case numbers have become less reliable.
Even if people infected with the coronavirus never seek testing or treatment, they excrete the virus in their feces. Because of this, monitoring wastewater is a useful way to monitor how much virus is spreading in a community. At the height of the pandemic, wastewater data provided early warning of impending surges and helped experts track the spread of new variants.
Now that the emergency phase of the pandemic is over, wastewater monitoring is an even more important tool, experts say. This spring, the Centers for Disease Control and Prevention stopped calculating community Covid levels and collecting some other types of surveillance data.
Although hospitalization rates have decreased, the virus still poses risks, especially for 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.
Background: Wastewater monitoring has expanded since 2020.
In September 2020, the CDC established a National Wastewater Surveillance System to centralize and standardize local efforts to track coronavirus in sewage. Since then, the system has expanded to cover more than 1,000 sampling sites across the country.
For the new study, researchers analyzed publicly available wastewater data for 268 counties participating in the national monitoring system. They compared wastewater trends in each county to local case and hospitalization rates in the first three quarters of 2022.
From January to March, a period that overlapped with the winter Omicron wave, high virus levels in wastewater closely mirrored high case and hospitalization rates, the scientists found. Wastewater levels are lower during April to June, before rising again during July to September. But in these last three months, when wastewater levels are high, official case and hospitalization rates have remained relatively low.
“Even though we saw a bump in 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 thought that wastewater is probably the more accurate approximation of what’s actually happening in terms of the emergence of Covid in communities across the country.”
What We Don’t Know: Viral evolution is a wild card.
The coronavirus continues to evolve. “We assume that the nature of the virus has not changed significantly in the way it is released and in the way 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 go down the drain more or less than previous variants.
Population susceptibility will also change, as people experience additional infections, new vaccines are launched, and immunity wanes. These changes can make it difficult to predict what future increases in wastewater levels will mean for public health.
The wastewater data will remain most useful when viewed alongside other public health metrics, the researchers said.