COVID-19 cases may be increasing again in New York City, new wastewater data hints.
Nearly all of the 14 wastewater treatments plants in the city are currently in the "high" detection level category, meaning likely 50 or more cases per 100,000 people, according to the NYS Wastewater Surveillance Network dashboard, which was updated Friday.
What's more, two-week trend data shows that just three of the plants are seeing decreases in virus detection.
Reported case numbers have not jumped. But fewer people are getting tested than earlier in the pandemic, so case numbers are a less reliable indicator of COVID-19 spread.
Health experts told ABC News that wastewater tracking is a good early detection tool for monitoring potential future upticks, but it's too early to determine what it means.
"Wastewater data can be a very helpful indicator of what is taking place," said Dr. John Brownstein, an epidemiologist and chief innovation office at Boston Children's Hospital and an ABC News contributor. "Now, it's an indication of the level of virus that people might be shedding, but it's not 100% related to the amount of illness people are experiencing."
He continued, "Because with the significant amount of immunity from previous infections, from vaccination, there may be community transmission, but it may be asymptomatic and so what wastewater can't tell you is actually the severity of the cases in the community."'
Wastewater surveillance checks for the virus in sewage -- from households and buildings -- that contains human fecal matter.
Because at least half of COVID-19 patients shed genetic material from the virus, or viral RNA, in their feces, the same tests that can determine if someone is positive can also detect the virus in wastewater samples.
When people shed the virus in their stool, they are often in the early stages of the infection, meanings levels of the virus in wastewater samples will often occur before the number of cases rise.
Reported COVID-19 cases have remained relatively flat since mid-April at about 270 cases for a seven-day rolling average, according to data from the city's Department of Health and Mental Hygiene. Hospitalizations and deaths have also remained relatively flat.
"Wastewater data is...unbiased," Dr. Peter Chin-Hong, an infectious diseases specialist at the University of California, San Francisco, told ABC News. "It just depends on the amount of virus circulating in the community, so although the absolute number might be debatable, that trend is always something that makes us pay attention."
However, he cautioned not to panic and said just because there are areas of the city in the "high" category, that does not mean a surge is imminent.
While the end of mitigation measures such as masking and some waning immunity may play a role if cases increase, Chin-Hong said the U.S. is in a much better position than it was in years past.
"In the old days, we just kind of had to be subject to the whims of the virus, but now we have weapons to fight back," he said. "And even if vulnerable populations are infected, they could avert or prevent hospitalization and death by taking advantage of these early, antivirals taken early in the course of disease, like Paxlovid and remdesivir and molnupiravir."
The experts said it's natural that wastewater data is going to ebb and flow and it is not going to stay low all the time. However, because everyone has a different risk tolerance, the data can help people decide if they want to modify their behavior.
"Knowing that viruses circulating may inform people about the kinds of activities you want to take part in," Brownstein said. "So just like you bring an umbrella for rain, you may change your behavior if you know that viruses are circulating. This is not a reason to panic. But more information can help empower people to make their own decisions."