For the first time in months, the COVID-19 transmission risk has increased from “low” to “moderate” in the four communities involved in the Contact Tracing Collaborative, which consists of Greenfield, Deerfield, Montague and Sunderland.

People with an active COVID-19 infection excrete the virus in their stool. As such, COVID-19 levels in wastewater across the collaborative are trending upward, according to Greenfield Health Director Jennifer Hoffman. Greenfield is among the western Massachusetts communities to participate in a testing program to track the concentration of COVID-19 in the wastewater system.

Additionally, there were seven identified cluster infections (of three or more people at one site) last week compared to one the week before, and average daily hospitalizations are at roughly one, though Baystate Franklin Medical Center saw a high of three hospitalizations in one day last week. The positivity rate also increased to 6.4% last week compared to 3.5% the week before.

Hoffman noted all nursing home facilities have been affected by the increase in cases as well.

As a result, the municipalities are in the yellow zone, with a “moderate” risk of COVID-19 transmission. Masking is encouraged.

In combination with wastewater testing, the four municipalities factor in hospitalization numbers, positivity rates and case numbers to determine COVID-19 risk levels.

Meanwhile, health officials continue to talk with nursing homes, businesses and schools to track cluster infections. Members of the collaborative added school absenteeism as an additional measure to compensate for the loss of PCR testing locally.

The following three zones, defined by the Contact Tracing Collaborative, are used to gauge risk levels of COVID-19 transmission in the four communities.

For the risk level in the area to change, at least two measures need to apply.

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.