hinese health authorities are ordering the use of controversial but more precise anal swabs to detect COVID-19 for a third year running as an Omicron cluster in Beijing grew to three people as of Wednesday.

After the city detected its first case of the highly transmittable variant on Saturday, The Beijing News said at least 27 anal tests had been administered in the apartment complex where the infected woman lives. Health workers also conducted extensive environmental swabs to trace the source of the transmission, the newspaper reported.

China has deployed anal swabs in major cities including Beijing and Shanghai since 2020. In the capital, the method was used in combination with antibody tests as well as throat and nasal swabs to screen millions last winter. While the approach isn’t considered practical for tracking larger clusters—an established method known as “pooling” is preferred—anal swabs are thought to be more accurate, especially in COVID hotspots.