GREENFIELD — The COVID-19 transmission risk has increased from “moderate” to “high” in the four communities involved in the Contact Tracing Collaborative, which consists of Greenfield, Deerfield, Montague and Sunderland.

Wastewater testing results showed there were 2.19 million copies of the virus per liter of wastewater last week compared to 654,600 copies per liter the week before. The four towns 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.

Case incidence has also increased substantially, though cluster infections (of three or more people at one site) have decreased slightly, from seven to five. Average daily hospitalizations remain roughly the same, at three.

The positivity rate increased from 6.3% to 8.15% between the two weeks.

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.

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.