This came after former Hospital Authority chief Leung Pak-yin said it is a waste of resources to adopt the tactic during the height of the epidemic, saying confirmed patients isolating at home will also excrete the virus.
But on an RTHK show on Monday, Wong said the authorities take into account infected people they know about.
“We won’t just check if the sewage samples test positive or negative, we will also look at the virus concentration,” he said.
“According to our experience, the concentration from recovered patients who’ve returned home is relatively low… When we consider whether to lock down high-risk buildings, we will also see if they have residents who recently tested positive through rapid tests. We won’t double count,” he said.
“So when sewage samples have a high virus concentration, we’re not just looking at patients we already know about… This is an effective way of finding invisible patients living in high-risk buildings.”
The environment chief said that during lockdowns in the past month, an average of more than 10 percent of residents tested each time turned out to be infected.
He added officials are making good use of resources by only locking down buildings with the highest risk.
