In encouraging sign, levels of coronavirus in Eastern Mass. waste water have dipped in recent days

Massachusetts waste water was tested at the Deer Island treatment plant and was found to have lower levels of the deadly coronavirus than in past weeks, which signaled the possibility of an end to the recent wave of COVID-19 in the state.David L. Ryan/Globe Staff

Is it just a blip or the beginning of a much-anticipated trend? The levels of coronavirus detected in Eastern Massachusetts waste water have turned downward in recent days, raising the possibility that a weekslong COVID-19 wave could be weakening.

As of Monday, the levels had dropped for six straight days for the communities in the southern section of the Massachusetts Water Resources Authority system and five of six days for the northern section.

Officials say waste water virus data can be an important early warning signal, detecting COVID-19 infections before people get tested and the tests are officially reported. As more people are using rapid at-home tests, whose results are usually not reported to state public health officials, waste water testing has become a key indicator of the virus’s prevalence.

“We’re hopefully headed down, but I think it’s not clear yet that we’re on the far side of the peak,” said Dr. Jonathan Levy, who chairs the department of environmental health at Boston University’s School of Public Health. “It’s a time for watchful waiting.

“We should remain cautious. Obviously, it’s graduation and prom season right now so there’s definitely exposure opportunities out there,” he said. “We should be mindful, keep an eye on things, and hope for continued improvement but also know that it’s partly in our control. The next couple of weeks will certainly be telling.”

Dr. William Hanage, an epidemiology professor at the Harvard T. H. Chan School of Public Health who is also a scientific adviser to Cambridge-based Biobot Analytics, the company that conducts the testing for the MWRA, tweeted Wednesday that “something has definitely changed in the Southern system (a similar though less pronounced drop is visible in the north). It may not be sustained, given many opportunities for transmission during graduation season, but it is welcome.”

Boston Wastewater watch - something has definitely changed in the Southern system (a similar though less pronounced drop is visible in the north). It may not be sustained, given many opportunities for transmission during graduation season, but it is welcome pic.twitter.com/jQ9eR307Xp

— Bill Hanage (@BillHanage) May 24, 2022

The decreases come as the state’s confirmed reported COVID-19 case numbers have also begun moving downward.

The US Centers for Disease Control and Prevention says that 10 of the state’s 14 counties have high community levels of COVID-19. The CDC recommends that, when virus levels are high, people should mask in indoor public spaces.

Experts have raised concerns that the arrival of the Omicron subvariants BA.2 and BA.2.12.1, were breathing new life into the pandemic at a time that the public was trying to return to normal after more than two years of pandemic disruption.

But some models have offered hope that the current Massachusetts wave could crest soon. Experts think that would happen for a variety of reasons, including the immune protection people have gotten from vaccinations and previous infections, and the warmer weather.

Early this year, the Eastern Massachusetts waste water levels dropped precipitously from their peak during the deadly surge caused by the original version of Omicron. The levels bottomed out in early March, then began rising again. The ominous rise was interrupted by a dip last month, but resumed until May 17, when the numbers dipped again.

The numbers have remained at much lower levels than they were during the Omicron surge.

Waste water from 43 communities, including Boston, converges at the MWRA’s Deer Island plant on Boston Harbor for treatment before being piped miles into the ocean. The water is tested for traces of the deadly virus. The MWRA reports numbers for both the southern and northern regions of its system. The testing determines the number of SARS-CoV-2 RNA copies per milliliter of waste water.

In the northern MWRA region, the seven-day average was 1,035 RNA copies/mL as of Monday. The number had gone as high as 1,273 on May 17, but it then dropped for five of the next six days.

In the southern region, the seven-day average was 925 RNA copies/mL on Monday. The number had gone as high as 1,332 on May 17, but it then dropped six days in a row.