Why Some Bigger MA Cities Have Lower COVID Wastewater Levels

BOSTON, MA — Since the COVID-19 pandemic hit in early 2020, cities have often become hotspots for the virus, and for good reason: more people means more potential for virus spread.

But now, in 2023, oftentimes a bigger population does not necessarily mean a higher infection rate, and the same is true for levels of the virus in wastewater. Why is that?

Though COVID-19 wastewater levels across Massachusetts have generally trended downward after reaching their highest point in about a year in early January — the most recent data shows levels in almost all areas are on a downtick —levels are still higher in many parts of the state than they are in Boston, its most populated city. For example, though virus wastewater trends in Peabody and Upper Blackstone tend to generally align with those of Boston as far as dips and spikes, levels are typically considerably higher in those smaller areas of the state than they are in its capital.

More Than One Factor

Acknowledging that "generally speaking, higher levels of COVID-19 particles in wastewater samples suggest higher levels of COVID-19 transmission," the Boston Public Health Commission told Patch that there are many actors aside from population density that can affect these levels.

"Very rarely is there a singular reason why one community is experiencing higher COVID-19 rates than others, but some factors include residents’ living and working conditions, access and adherence to high-quality masks/respirators, and prior immunity due to vaccination/booster coverage and prior infection," the agency told Patch.

A representative for Biobot, the first company in the world to commercialize data from sewage, echoed these sentiments, saying that "While big urban centers like New York City or Boston can have higher rates of COVID-19, this is not always the case."

"From a public health perspective, variation in vaccination levels as well as levels of circulating variants within each community can impact overall infection rates across communities," Biobot added. "Additionally, it is possible that the number of COVID-susceptible individuals is lower in Boston right now than in areas outside of Boston."

Smaller Systems Show More Variability

Speaking on background, another expert told Patch that the architecture and composition of each individual wastewater collection system can affect the numbers, with "smaller systems usually showing more variability."

"There is not a one-to-one correspondence between case numbers and wastewater levels," the source said. "Factors such as the geographical distribution, flow rates through the system, and other issues can affect both the viral levels in wastewater and their detection."

For that reason, multiple experts said it might be more helpful to note local wastewater COVID trends — or increases and decreases over time — rather than to track concentration per liter of the virus in your area compared to that of other parts of the state or country.

"Biobot accounts for varying population sizes with a normalization method that adjusts for differences in dilution and population size," the representative added. "This means that we can compare trends across urban and rural areas, such as Boston and other parts of Massachusetts."

Vaccination Is Still Key — And Free

As experts continue to emphasize that vaccination is the best way to prevent severe disease, the state of Massachusetts announced that a $75 gift card will be offered to any residents who get vaccinated (first dose, second dose, or booster) at select clinics through March 31, 2023, while supplies last.

“Offering free COVID-19 vaccines has been an indispensable part of our pandemic response in Boston and will become even more important as the national public health emergency ends,” Dr. Bisola Ojikutu, Executive Director of the Boston Public Health Commission and Commissioner of Public Health said in a news release earlier this month.

COVID-19 vaccines and boosters are free and no ID or health insurance is needed.