Weekly averages of COVID-19 detected in Boston wastewater reach highest levels yet

MWRA's Deer Island Sewage Treatment Plant is seen from Spectacle Island in Boston Harbor.Lane Turner/Globe Staff

The seven-day averages of the amount of coronavirus found in Boston-area wastewater this week reached their highest levels since the start of the pandemic.

The amount of virus detected in tests of water into the Massachusetts Water Resources Authority’s Deer Island treatment plant have been sharply rising since the beginning of December in both the north and south systems, and the weekly averages reported Tuesday surpass levels reached during last winter’s COVID-19 surge.

The seven-day average of copies per milliliter in the southern system, which includes communities south of Boston, reached 2,104 on Monday, surpassing the highest average reached during last winter’s COVID-19 surge. Since the data was last updated on Friday, the average RNA levels recorded in the southern system surpassed the one-time highest average of 1,476 copies per milliliter reached on Jan. 11 several times: the seven-day average stood at 1,749 on Dec. 4 and 1,862 on Dec. 5 ahead of Monday’s reading.

The southern system also this week reached its second-highest all-time daily high, with 3,480 copies per milliliter recorded on Saturday. That number follows the highest recorded value of 3,772 copies reported on Jan. 25.

The seven-day average of RNA copies per milliliter in the northern system, which includes Boston and communities north of the city, was 1,244 on Monday, its highest level yet. This week, the single-day readings of the amount of virus detected in the north system also reached its highest-ever reading, with 1,905 RNA copies per milliliter recorded on Sunday.

The tests that look for RNA copies per milliliter of wastewater at the treatment plant are intended to be an early warning system for COVID-19 surges. Cambridge-based Biobot Analytics, which analyzes the samples, says it has found that the amount of virus in the waste water is correlated with newly diagnosed coronavirus cases four to 10 days later.

The record-high averages come as COVID-19 cases have risen in Massachusetts, in what officials fear could reflect a winter surge in the Northeast as people spend more time indoors and immunity from vaccinations wane.

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Martin Finucane of the Globe Staff contributed to this report.