After declining for weeks following a winter spike, the amount of COVID-19 detected in Boston-area waste water has begun to tick up slightly, according to data from the Massachusetts Water Resources Authority — though an infectious disease expert said that does not necessarily signal an upcoming rise in cases.
The amount of COVID-19 detected in Boston-area sewage had been decreasing since early January after reaching a peak following the holidays. But from March 16 to March 21, the average amount of coronavirus found in the southern system increased by 39 percent, while the amount of virus detected in the northern system increased by 30 percent.
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The increase is “unexpected” given the recent stability in COVID-19 cases in Massachusetts, said Dr. Shira Doron, chief infection control officer for Tufts Medicine health system and hospital epidemiologist at Tufts Medical Center.
But, she cautioned, it’s too early to tell whether the elevated amounts predict an upcoming rise in cases.
A number of reasons could be behind the increase, she said.
The rise could be the result of more COVID circulating, or a brief uptick that will soon go down, a cycle that has happened before and can be expected “for the rest of time” as the virus remains, Doron said.
Another explanation could be weather patterns and rainstorms, which can create more runoff into the waste water collection system from animals that have COVID, Doron said.
Plus, different variants can have different levels of viral loads, potentially resulting in higher amounts of COVID in waste water even if the number of people infected remains level. Medications, treatments for COVID, and how recently a person was vaccinated can also affect how much COVID is in a person’s system.
“There’s so many different factors that go into how much virus ends up in the waste water, besides how many people are infected,” Doron said. “Time will tell if this ends up being representative of a real thing. More cases? Do we have more tourists? Do we have more gatherings? We need to see how it plays out.”
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Doron said she is not seeing more COVID in patients at Tufts. In fact, she said, it’s “close to the lowest levels we’ve ever seen.”
The water samples are taken from the MWRA’s Deer Island sewage treatment plant three to seven times per week and are analyzed by Biobot Analytics of Cambridge, according to the MWRA website. The data serves as a tool in assessing the local prevalence of the virus and can signal changes in COVID-19 trends ahead of hospitalization or case count reports.
The north system includes Boston, Cambridge, Somerville, Chelsea, and towns extending to Wilmington. The south system includes Quincy, Newton, Brookline, and a stretch of towns that spans Framingham to Hingham.
The slight increase comes after the level of virus in Boston-area waste water has been gradually declining since reaching a winter peak in early January. This winter’s peak was dramatically lower than one the previous year, when the highly transmissible Omicron variant sent COVID cases soaring.
The level of virus in waste water in the northern system has declined about 79 percent since it reached a peak on Jan. 1. And in the southern system, the amount of COVID has also declined about 79 percent since reaching a winter high on Jan. 5.
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Shortly after the winter spike, the Boston Public Health Commission announced an initiative to sample waste water in neighborhoods across the city in an effort to better understand how the virus is spreading and mount responses in communities that are experiencing increasing rates of the virus.
The numbers revealed stark disparities, with Roxbury seeing the city’s highest coronavirus levels, more than three times those of Roslindale/West Roxbury and Charlestown, which had the lowest levels.
The slight uptick in COVID-19 waste water data also comes just days after the three-year mark of the date when the World Health Organization declared COVID-19 a pandemic on March 11, 2020.
Globe correspondent Nick Stoico and Zeina Mohammed of the Globe staff contributed to this report. Clarification: An earlier version of the story described Dr. Shira Doron as the chief infection control officer at Tufts Medical Center. Doron’s role applies to Tufts Medical health system.
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Amanda Kaufman can be reached at amanda.kaufman@globe.com. Follow her on Twitter @amandakauf1.