Coronavirus rates have risen at 38% of U.S. wastewater sampling sites tracked by the Centers for Disease Control and Prevention over the last two weeks, possibly warning of new Covid-19 upticks in some parts of the country even as the number of positive tests plummets nationwide.
Some 2% of the 688 wastewater sites tracked by the CDC showed a 1-9% increase in coronavirus levels from February 27 to March 13, while 9% of sites showed a 10-99% increase, 12% showed a 100-999% increase and 15% showed an increase of 1,000% or greater.
During the same period, the number of daily Covid-19 infections diagnosed through standard testing dropped more than 45% nationwide, reaching an average of 34,113—an eight-month low—in the weeklong period ending Sunday, according to CDC data, and every state has reported a drop in daily infections of 13% or more according to an analysis by New York Times.
In some areas, neighboring wastewater sampling sites show contradictory readings: Several sites in the Chicago area reported coronavirus levels had fallen by up to 99% from February 27 to March 13, but one showed they had leapt by at least 1,000%.
The New York City and Houston areas and Miami-Dade County also gave conflicting reports of rising or falling coronavirus levels in wastewater, a disparity Bloomberg said was possibly due to small-scale outbreaks that had not grown large enough to be picked up at multiple sites.
Coronavirus genetic material can be detected in the sewage of people infected with the coronavirus, including people without symptoms. As a result, hundreds of wastewater treatment plants test for the virus, providing an early warning on coronavirus trends. A wastewater surveillance system installed at the University of California San Diego led to early diagnosis of 84.5% of all Covid-19 cases on campus from November to December 2020, according to a study published in August by the American Society for Microbiology. Though it’s too early to tell if the recent spate of rising Covid-19 wastewater numbers heralds a new surge in infections, Dr. Amy Kirby, head of the CDC’s National Wastewater Surveillance System, told Bloomberg community health officials should keep a close eye on wastewater data and use it as an “early warning sign” if numbers continue to increase. Kirby said increasing levels of Covid-19 detected at wastewater sampling sites could reflect a bump in infections due to changing Covid-19 safety guidelines, such as the abandoning of mask mandates by all state governments as infection numbers subsided in January following a massive omicron-driven surge.
Some parts of Europe reported a spike in Covid-19 cases in February, shortly after countries including Denmark, Finland, Norway and Sweden, scrapped almost all Covid-19 restrictions. Because Covid-19 trends in Europe tend to foreshadow those in the United States by a few weeks, the U.S. could also experience a rise in infections, City University of New York virologist John Dennehy said. “We might want to be done with COVID-19 but the virus is not done with us,” Dennehy wrote in an email. “I am concerned that a new variant of concern will arise in the coming months.”
The CDC’s National Wastewater Surveillance System has limitations. It can’t capture data from homes with their own septic systems, sewage plants that treat wastewater before it arrives or facilities like prisons or hospitals that treat their own waste. Wastewater data also can’t be used to precisely gauge the number of infected individuals in a community, the CDC said. Additionally, wastewater sampling sites tracked by the CDC are not evenly distributed across the United States. Missouri, Ohio and Illinois all have more than 70 sites, while states like Alabama, New Jersey and New Mexico have none.
Though Covid-19 can be detected in wastewater, there’s no evidence that anyone has ever contracted Covid-19 from wastewater, the CDC said.