Eastern Mass. waste water coronavirus levels declining slowly

The white tanks of the MWRA's Deer Island treatment plant seen from a distance. Waste water is tested at the facility for traces of the virus that causes COVID-19.Lane Turner/Globe Staff

The levels of coronavirus detected in Eastern Massachusetts waste water fluctuated up and down in the week ending Tuesday, rather than making a much hoped-for race to the bottom, according to data from the Massachusetts Water Resources Authority.

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.

Other pandemic metrics, including cases and hospitalizations, have been on the decline.

Pandemic models have suggested that COVID-19 infections in Massachusetts, which had been rising for weeks, would peak before summer and then drop. Experts said they thought that would happen for a variety of reasons, including the immune protection people had gotten from vaccinations and previous infections, and the arrival of warmer weather. The emergence of the new subvariants BA.4 and BA.5, however, has injected a note of uncertainty about how fast the numbers might fall.

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 sections of its system. The testing determines the number of SARS-CoV-2 RNA copies per milliliter of waste water.

In the northern MWRA section, the seven-day average ranged from 605 to 724 RNA copies/mL over seven days, ending with a two-day uptick to 696 on Tuesday. The number had gone as high as 1,273 on May 17 during a spring bump in infections.

In the southern section, the seven-day average ranged from 610 to 745 copies/mL over the same period, ending with a two-day downtick to 615. The number had gone as high as 1,332 on May 17.

After the devastating Omicron surge early this year, levels plummeted. They bottomed out in early March before beginning to rise slowly again toward their mid-May peaks.