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

The collaborative is reporting eight active cluster infections (of three or more people at one site) last week compared to six the week before.

The positivity rate, meanwhile, decreased from 10.6% to 5.6%, average daily hospitalizations decreased from 2.7 to 1.6 and the case incidence decreased.

In wastewater testing, there were 688.2 copies of the virus per liter last week compared to 1.02 million copies per liter the week before. Greenfield is 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 COVID-19 infection excrete the virus in their stool.

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 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.