The amount of coronavirus found in the waste water coming from Boston and some surrounding communities has ticked down in the past few days after weeks of increases, though levels continue to rise in other towns served by the Massachusetts Water Resources Authority.
The tests of water coming into the MWRA’s Deer Island treatment plant are intended to be an early warning system for surges of the virus. The new results sent mixed signals about where the pandemic is headed.
The numbers overall have bounced up from extremely low levels reached in June to levels comparable to those in April. They are still much lower, though, than the peak reached early this year.
The testing program looks for SARS-CoV-2 RNA copies per milliliter of waste water. Cambridge-based Biobot Analytics, which conducts the testing, 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 authority reports two numbers, one for samples taken in the northern section of the MWRA system, which includes Boston, and the other for samples taken in the section that serves communities to the south.
The numbers for the northern section rose through late last week then ticked down over the past few days. The numbers for the southern section, which covers communities south and west of the city, have continued to move upward.
The MWRA updated the numbers Tuesday afternoon to reflect tests taken as recently as Monday.
The numbers come as the state’s coronavirus case numbers have risen and officials are imploring everyone to get vaccinated to protect themselves and others. Some other countries that have seen a surge from the variant are now seeing numbers go down again, offering a glimmer of hope.
Joseph Allen, an associate professor at Harvard University’s T.H. Chan School of Public Health, said the waste water coronavirus testing has been “one of the must useful early indicators of where we’ve been headed — through the late fall, winter and spring.”
He noted, however, that experts believe that, thanks to vaccinations, future case increases won’t be accompanied by commensurate surges in hospitalizations and deaths.
Waste water and case data are expected to “decouple” from hospitalization and death data as the vaccines protect people who contract the virus from getting severely ill, he said.
“Massachusetts will be a bellwether for how the highly vaccinated states do against the variant,” he said.
Martin Finucane can be reached at firstname.lastname@example.org.