Wastewater testing results across the four municipalities in the Contact Tracing Collaborative — which includes Greenfield, Deerfield, Montague and Sunderland — have increased nearly four-fold compared to the last several samples, according to regional epidemiologist Jack Sullivan.

“It’s certainly not as high as it’s been during the peak, but we’re seeing an uptick again on a fairly regular seasonal basis,” Sullivan explained. “We’re starting to see that uptick now with the wastewater and with the positivity rate, both in our region and around the state.”

The four municipalities are among the western Massachusetts communities to participate in a testing program to track the concentration of COVID-19 in the wastewater system, as people with an active infection excrete the virus in their stool.

With more than 1 million copies of the virus per liter, Sullivan explained that on a scale of low to substantial, all four municipalities saw wastewater results last week that are considered to be substantial, both by local public health officials and Biobot Analytics, the Cambridge-based company that analyzes wastewater samples. Last week, there was a weighted average across all four communities of 1.23 million copies of the virus per liter.

Cluster infections (of three or more people at one site) increased from three to five between last week and the week prior. The positivity rate increased from 4.12% to 7.47%, while hospitalizations and case incidence have stayed steady.

Sullivan, who is also the regional epidemiologist for Blackstone Valley Partnership for Public Health in the Worcester County area, said other regions are seeing similar trends in data.

“We’re seeing similar numbers down there, both in wastewater and positivity, and right now those are our two best gauges,” he said.

Comparing this winter’s data to past years, however, is a bit more challenging, according to Sullivan.

“We weren’t doing wastewater testing the first two years and we were doing PCR testing,” he said. “Now, we don’t have PCR and most of the testing is done at home, [with] rapid tests. We do have wastewater data. It’s a little bit apples to oranges as far as trying to compare it.”

As for other respiratory illnesses, such as flu or respiratory syncytial virus (RSV), the community is also seeing an uptick, Sullivan said. Those are tracked in part by emergency room visits and hospitalizations, given RSV isn’t among the illnesses tracked by the state’s system for tracking infectious diseases.

With Hanukkah already underway and Christmas and New Year’s Day coming up, Sullivan said his recommendation is “just to be smart” as gatherings take place.

“A lot of people have COVID fatigue, and I absolutely get that,” he said.

Sullivan said vaccinations and wearing a mask are two of the best ways to reduce transmission and prevent serious illness.

“I certainly understand there are people who are not open to that,” he said. “I hope they’ll take care of themselves in other ways.”

In combination with wastewater testing, the four municipalities factor in hospitalization numbers, positivity rates and case numbers to determine COVID-19 risk levels. The following three zones, defined by the Contact Tracing Collaborative, are used to gauge risk levels of COVID-19 transmission locally. For the risk level to change, at least two measures need to apply.

The four municipalities are currently in the red zone, with a high risk of COVID-19 transmission. Masking is recommended.

Red zone: High COVID-19 risk

Masking recommended.

■Cluster infections (of three or more people at one site): five or more.

■Percent positive tests out of total tests performed: greater than 10%.

■Hospitalizations: five or more.

■Increase in case trend.

■Increase in wastewater trend.

Yellow zone: Moderate COVID-19 risk

Masking encouraged.

■One to four cluster infections.

■Stable positive testing.

■Fewer than five hospitalizations.

■Stable case incidence.

■Stable wastewater.

Green zone: Low to minimal COVID-19 risk

Masking optional.

■No cluster infections.

■Decrease in positive tests.

■Decrease in hospitalizations.

■Decrease in case incidence.

■Decreased virus copies in wastewater.

Reporter Mary Byrne can be reached at mbyrne@recorder.com or 413-930-4429. Twitter (X): @MaryEByrne.