Wastewater data has become perhaps the best metric to track the wave of COVID in the U.S., but it’s an imperfect tool.

“We are seeing rates are going up across the country,” said Amy Kirby, program lead for the C.D.C.’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 — like those who are older, immunocompromised or already have a serious illness — it’s become a crucial tool helping them understand when to be particularly careful.