BEIJING (AP) — Authorities in a Chinese city bordering Myanmar locked down the city Wednesday, shutting most businesses and requiring residents to stay at home, as a fresh outbreak of COVID-19 expanded.

Another 15 cases were found in Ruili in the 24-hour period ending at midnight, on top of six in the first two days, health authorities in southwest China's Yunnan prefecture said.

The lockdown, effective at midnight, shut down all businesses and public institutions except hospitals, pharmacies and essential shops like grocery stores. It affects the urban area of Ruili.

The latest cases were discovered during mass testing of residents. The positive cases include both Chinese and Myanmar nationals in the city, where there is an active cross-border trade. Authorities said they would step up border controls.

Ruili previously had a COVID outbreak in March and launched a campaign to vaccinate the entire city in April.

China has relied on a tough lockdown strategy and mass testing to tamp down outbreaks, even as it has stepped up the pace of vaccinations. Central health officials have said they want to vaccinate 80% of the population.