Hong Kong will try to test around 860 people living in two apartment buildings for coronavirus, after a third locally transmitted case of Covid-19 was identified as part of a new cluster in the city.

On Wednesday, a 66-year-old woman and her five-year-old granddaughter were confirmed as the first two locally transmitted coronavirus cases in 22 days. The pair do not live together.

The new case, confirmed on Thursday, is the 66-year-old woman’s husband.

Hong Kong’s Housing Authority said it would distribute deep throat saliva sample kits to all residents living in the same buildings as the couple and their granddaughter by the end of Thursday.

Participation in the testing is voluntary according to Dr. Chuang Shuk Kwan from the city’s Center for Health Protection.