This chart may tell us more about the Mass. COVID surge than any other. Here’s why

The Omicron variant now accounts for 95.4 percent of COVID-19 cases in the United States, according to data released Tuesday by the CDC. COVID cases in Massachusetts have skyrocketed in recent weeks, breaking previous records, and hospitalizations have risen to levels near last winter’s surge, though so far deaths have risen more slowly.

But there’s another COVID-19 metric experts are monitoring, one that makes clear just how startlingly widespread the virus is now: waste water — or sewage — surveillance data. And it has raised alarms about where we’re headed in the coming weeks.

Here’s how it works: When people have COVID-19, they shed the coronavirus that causes it, SARS-CoV-2, not only from their mouths and noses, but in fecal matter, which gets flushed down the toilet and into area waste water.

In the Boston area, waste water from 43 communities, including Boston, that belong to the Massachusetts Water Resources Authority system, converges on the Deer Island treatment plant in Boston Harbor.

There, the sewage is tested to determine the number of SARS-CoV-2 RNA copies per milliliter of waste water. More virus means more cases of COVID-19 in the region.

The testing picks up COVID-19 whether people in the community have symptoms, mild symptoms, or no symptoms at all, said Newsha Ghaeli, president and co-founder of Cambridge-based Biobot Analytics, which has conducted the tests since early in the pandemic. It doesn’t matter if people are vaccinated or unvaccinated, or what variant they have. The waste water readings provide perhaps the clearest pictures of just how widely COVID is circulating.

What are the tests saying now?

In Massachusetts, where numbers have taken off like a rocket, the MWRA waste water testing numbers are broken down into two groups of communities, those in the northern region and the southern region. Twenty-two communities are in the northern region, 17 are in the southern, and four, including Boston, are in both.

In the southern region, a seven-day average of 11,466 RNA copies/mL — or bits of virus per milliliter — was detected as of Monday, more than eight times higher than levels reached in mid-December when the Omicron variant’s impact was still unknown. The northern region has seen a rise to a seven-day average of 8,353 RNA copies/mL, a nearly seven-fold increase, over the same period.

The numbers are also far higher than ever reached during last winter’s COVID surge, when the southern region topped out at 1,476 RNA copies/mL and the northern region at 1,130.

The swiftly climbing numbers suggest more case increases could be ahead. The crucial question remains how many of those cases will result in hospitalizations and deaths, experts say.

The waste water data mean “that a huge number of people in the community are sick with COVID,” at least to some extent, said Andrew Lover, an assistant professor of epidemiology in the School of Public Health and Health Sciences at UMass Amherst.

“Everyone should definitely think about minimizing social contacts” and taking precautionary measures such as getting vaccinated and boosted, practicing social distancing, and improving ventilation, Lover said. He also said people should mask in indoor public spaces, and he supported a statewide indoor mask mandate. “With the way hospitalizations are going, we need every tool we can get,” he said.

He said people should keep a close eye on the hospitalization and death numbers, noting that those two measures lag behind waste water and case numbers. Even if “we magically implemented complete COVID control tomorrow morning we would still see rising hospitalizations and deaths for several weeks,” he said.

Matthew Fox, a professor of epidemiology and global health at Boston University School of Public Health, said, “waste water data has proved a very useful early indicator of what is going to happen to cases. So I think we can expect that cases are going to increase over the next week or two. This isn’t a surprise given how fast omicron moves and how much contact people have been having over the holidays.”

Fox said Omicron could produce a “short and intense surge” and he hoped that by the end of January “we will be in a much better place.”

Fox and Lover both said they’re concerned that, even if Omicron proves to be less severe, a tsunami of cases could result in serious cases overburdening the health care system.

“While Omicron is a milder form of COVID,” Fox said in an e-mail, “the sheer numbers of people who will get it means that our hospitals may become overwhelmed.”

Waste water testing can provide an early warning of several days for case increases, said Biobot’s epidemiology group lead, Scott Olesen. “What waste water testing is doing is it’s measuring the number of people infected now,” many of whom will be tallied as cases in the future, he said.

Ghaeli said waste water testing has become particularly important at this stage in the pandemic when many people are feeling mild symptoms, using at-home tests, and isolating at home, and not necessarily reporting their case to anybody and thus not being reflected in any official case counts.

Biobot, which was founded in 2017, has done testing in all 50 states and several territories, Ghaeli said. It currently has contracts with water systems in more than 20 states.

“We’re definitely seeing a nationwide surge right now,” she said.