Coronavirus levels in Eastern Massachusetts waste water are sending mixed signals, with numbers dropping precipitously in one area but not in another.
The seven-day average virus level in the northern section of the Massachusetts Water Resources Authority system began a steep decline in the past few days, an encouraging sign that the pandemic might be weakening. But the level in the southern section remained essentially flat.
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.
The latest numbers reflected waste water tests taken up until Tuesday.
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 advent of warmer weather. The arrival 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 regions 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 dropped to 592 RNA copies/mL as of Tuesday. The number had gone as high as 1,273 on May 17.
In the southern section, the seven-day average was 742 RNA copies/mL as of Tuesday. The number had gone as high as 1,332 on May 17. The southern section numbers have been fluctuating in a range between 704 and 770 since May 29.
After the devastating Omicron surge early this year, levels plummeted. They bottomed out in early March before beginning to rise slowly again toward the latest peak in mid-May.
Material from prior Globe stories was used in this report.