Traces of coronavirus in wastewater at Deer Island plant tick down

A coronavirus testing site last week at Quinsigamond Community College in WorcesterErin Clark/Globe Staff

Traces of the coronavirus in the wastewater flowing into the MWRA’s Deer Island treatment plant ticked downward at the end of last week, while remaining at higher levels than during the spring.

Under a pilot program, tests are conducted for SARS-CoV-2 RNA copies per milliliter of wastewater. People shed the virus in their stool and it travels downstream to the treatment plant. State officials are hoping the tests can serve as an early warning system for virus surges.

Cambridge-based Biobot Analytics, which conducts the testing, says it has found that the amount of virus in the wastewater is correlated with newly diagnosed coronavirus cases 4 to 10 days later.

Like other coronavirus metrics monitored by the state, the virus detections in the wastewater have shown a major surge in the spring, a decline in the summer, and a rapid increase in the fall.

The data released on Saturday reflected tests conducted up to Friday.

More data that may be a cause for concern: Despite officials’ pleas that people not travel and gather for the holidays, the Transportation Security Administration reported that on Sunday, for the third consecutive day, more than 1 million people traveled through US airport checkpoints.