A wastewater network that monitors for Covid-19 trends is warning that cases are once again rising in many parts of the U.S., according to an analysis of Centers for Disease Control and Prevention data by Bloomberg.
More than a third of the CDC’s wastewater sample sites across the U.S. showed rising Covid-19 trends in the period ending March 1 to March 10, though reported cases have stayed near a recent low. The number of sites with rising signals of Covid-19 cases is nearly twice what it was during the Feb. 1 to Feb. 10 period, when the wave of omicron-variant cases was fading rapidly.
It’s not clear how many new infections the signs in the sewage represent and if they will turn into a new wave, or will be just a brief bump on the way down from the last one. In many parts of the country, people are returning back to offices and mask rules have been loosened — factors that can raise transmission. At the same time, warmer weather is allowing people to spend more time outside, and many people have recently been infected, which may offer at least temporary protection against getting sick again – factors which would keep cases down.
“While wastewater levels are generally very low across the board, we are seeing an uptick of sites reporting an increase,” Amy Kirby, the head of the CDC’s wastewater monitoring program, said in an email to Bloomberg. “These bumps may simply reflect minor increases from very low levels to still low levels. Some communities though may be starting to see an increase in Covid-19 infections, as prevention strategies in many states have changed in recent weeks.”
Bloomberg reviewed data for more than 530 sewage monitoring sites, looking at the most recent data reported during the 10-day window from March 1 to March 10. Out of those sites, 59% showed falling Covid-19 trends, 5% were roughly stable, and 36% were increasing. Rises or declines are measured over a 15-day period.
Fewer sites had data during the Feb. 1 to Feb. 10 window. During that period, 80% of sites showed a decreasing trend, 5% were stable, and 15% were rising.
Wastewater samples can’t tell how many people have Covid-19. Instead, they measure how much of the virus is being found in sewer water. A high concentration in a sample can indicate a rising number of infections, often days before those cases show up in tests.
Official case numbers determined through Covid tests have become increasingly unreliable. With wider access to at-home tests, many infections — particularly mild ones — are never reported. The proportion of unreported cases can be even higher in the middle of surges and at times when tests have been hard to come by.
In the greater New York City area, for example, there are signs of an increase. While most of the region’s sampling sites do not have recent data, a wastewater site in Fairfield, Connecticut, shows a high rate of increase. A site in Nassau County shows a moderate increase in Covid-19 detection.