For the first time since the beginning of December, the COVID-19 transmission risk for the four communities in the Contact Tracing Collaborative — Greenfield, Deerfield, Montague and Sunderland — has decreased from “high” to “moderate.”

Wastewater testing results last week showed there were 1.22 copies of the virus per liter of wastewater compared to 1.42 million copies per liter in the week prior. 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.

Cluster events (of three or more people at one site) decreased from five in the previous week down to four last week, the municipalities’ test positivity rate decreased from 9.56% to 7.66%, and case incidence decreased by nearly half. The daily rate of hospitalizations due to COVID-19, however, increased from 1.8 to 2.3.

Greenfield residents are able to get free at-home tests by visiting the Health Department’s offices at 20 Sanderson St. Deerfield and Montague residents can get tests by visiting their respective town halls, as long as supplies last. Leverett and Shutesbury also offer COVID-19 test kits at their respective town halls.

Additionally, tests can be ordered from the federal government for free by visiting covid.gov/tests.

In Greenfield, COVID-19 booster vaccinations, which aim to reduce the severity of COVID-19 symptoms, are available at CVS Pharmacy on Federal Street, Big Y on the Mohawk Trail, Rite Aid on Main Street, Stop & Shop on the French King Highway and Walgreens on Pierce Street.

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 of 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: Moderatoe 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.

■Decrease in wastewater virus copies.

Chris Larabee can be reached at clarabee@recorder.com.