U.S. Sewer Data Warns of a New Bump in COVID-19 Cases After Lull

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Photo: Micah Green/Bloomberg


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.

“It’s too early to know if this current trend will continue or whether we’ll see a corresponding increase in reported cases across the country,” said the CDC’s Kirby. “We encourage local health officials to monitor their numbers closely and use these data as an early warning sign if wastewater levels continue to increase.”

In some places, the signals are less than clear. In Miami-Dade County, for example, one sample site shows a decline in the amount of Covid found in wastewater. But two other sites for the county show an increase. It’s possible the data are finding the beginnings of a small, still-local cluster of cases. Or the data could be based on a relatively low level of virus found, exaggerating the size of the change because of the low baseline.

People infected with the coronavirus shed viral particles in their stool, which then flows into the sewer system when they use the toilet. Because they begin to give off the virus early in the course of infection, wastewater samples can identify a rising trend of infections early on.

So far, the warning given by the sewer networks hasn’t shown up in case numbers and the number of patients being hospitalized for Covid-19 is still near recent lows. The 65-years-and-older population in the U.S. — among the most vulnerable to severe Covid-19, hospitalization, and death — is also the country’s best-vaccinated demographic. Eighty-nine percent of that group is fully vaccinated, and 67% of those people have received a booster dose.

The increase in Covid wastewater warnings comes about two weeks after the CDC changed its recommendations to the country on masking and other public health measures. The agency revamped its recommendations to focus in part on making sure hospitals were not overwhelmed, and put much of the country in its “low” level of Covid risk.

According to the CDC’s latest community levels rating, 98% of the U.S. population is in places with a “low” community-level rating. Those ratings, however, are based on case numbers and hospitalizations. Wastewater data can often pick up a rise in cases several days before case numbers rise.

Cases are on the rise in several European countries after the continent went through a similar pattern to the U.S. Germany, Switzerland, and Austria, as well as other European countries, have all reported significant increases in case numbers over the last two weeks. U.S. trends often follow what happens in Europe, given the rough similarities in the climate, population, and public health approach. 

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