The curves on some COVID-19 graphs are looking quite steep, again.

Reported levels of the virus in U.S. wastewater are higher than they have been since the first omicron wave, according to data from the Centers for Disease Control and Prevention, although severe outcomes still remain rarer than in earlier pandemic winters.

“We are seeing rates are going up across the country,” said Amy Kirby, program lead for the CDC’s National Wastewater Surveillance System. The program now categorizes every state with available data at “high” or “very high” viral activity.

The surge might reach its peak this week or soon after, modelers predict, with high levels of transmission expected for at least another month beyond that.

Hospitalizations and deaths have remained far lower than in previous years. There were around 35,000 hospitalizations reported in the last week of December — down from 44,000 a year earlier — and 1,600 weekly deaths as of early December, down from 3,000. (At the same time in 2020, there were around 100,000 hospitalizations and 20,000 deaths each week.)

Many of the metrics used early in the pandemic have become much less useful indicators of how widely the virus is spreading, especially since federal officials stopped more comprehensive data tracking efforts when they declared an end to the public health emergency last spring. Higher population-wide immunity has meant fewer hospitalizations even with high virus spread, and the sharp decline of COVID test results reported to authorities has made case counts far less relevant.

Wastewater testing remains one of the few reliable instruments still available to monitor the virus. It can signal the start of a surge before hospitalizations begin to rise, and it includes even people who don’t know they have COVID. For many who remain at higher risk from the virus — including those who are older, immunocompromised or already have a serious illness — it has become a crucial tool in helping them understand when to be particularly careful.

But it’s an imperfect metric — useful primarily for identifying if there’s an acceleration of virus spread, not for telling you exactly how much virus is circulating. That’s why many scientists who study the data will say only that it shows the nation is in the middle of a large wave, not whether the surge this winter is bigger than previous ones.

This article originally appeared in The New York Times.