The Significance of Case Counts: Their Declining Reliability
The transmission of the coronavirus through stool has been discovered even among individuals who do not undergo testing or seek medical treatment. This insight has made wastewater surveillance an effective method to assess the extent of viral circulation within a community. During the peak of the pandemic, wastewater data played a critical role in providing early warnings for surges and aiding in the monitoring of the spread of new variants.
Even now that the emergency phase of the pandemic has subsided, experts emphasize the continued importance of wastewater surveillance. The Centers for Disease Control and Prevention (CDC) recently ceased calculating community Covid levels and collecting certain types of tracking data, making wastewater surveillance even more essential. With hospitalization rates decreasing but the virus still posing risks, wastewater data can assist individuals and institutions, such as long-term care facilities, in making better-informed decisions regarding the implementation of additional precautions.
The Evolution of Wastewater Surveillance since 2020
In September 2020, the CDC established the National Wastewater Surveillance System to streamline and standardize local efforts aimed at tracking the coronavirus in sewage. Since its inception, the system has expanded to include over 1,000 sampling sites across the United States.
For the present study, researchers analyzed publicly available wastewater data from 268 counties participating in the national surveillance system. They compared the wastewater trends in each county with the local case and hospitalization rates during the first three quarters of 2022.
The scientists found that during the period from January to March, which coincided with the winter Omicron wave, high levels of the virus in wastewater closely corresponded to elevated case and hospitalization rates. In the April-to-June timeframe, wastewater levels decreased, only to rise again in July-to-September. However, during these last three months, when wastewater levels were high, official case and hospitalization rates remained relatively low.
Dr. Meri Varkila, a postdoctoral fellow at Stanford University and one of the study’s authors, expressed, “Even though we saw a bump in the case rates, it wasn’t as significant as what we saw in the wastewater data. We concluded that wastewater is likely the more accurate estimate of actual Covid occurrence in communities nationwide.”
What Remains Unknown: The Role of Viral Evolution
The coronavirus is continuously evolving, and while it is assumed that its shedding behavior and environmental distribution remain relatively consistent, there may be future variants that demonstrate different patterns compared to past variants when present in sewage.
In addition, changes in the population’s susceptibility due to recurring infections, the introduction of new vaccines, and fluctuating levels of immunity may complicate the interpretation of future spikes in wastewater levels and their implications for public health.
Therefore, the researchers stress the importance of considering wastewater data alongside other metrics in the field of public health.
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