The admission comes six months after the novel coronavirus was declared a global health emergency by the World Health Organization. Since then, the virus has spread and infected more than 17 million people around the world and killed 667,808 — a number that is increasing daily.
More than 66 percent of China's quarantined residents are in the western province of Xinjiang. Officials there have locked down some residential areas, restricted public transit and ordered mass testing as authorities struggle to contain the flare-up.
The last time Beijing reported more than 18,000 people who had been in close contact with confirmed coronavirus cases was on April 3.
At the height of the pandemic, the number ballooned to 189,000, according to China's National Health Commission.
On Wednesday, China reported 105 new cases of COVID-19.
But the upward trend is not taking place only in China.
Australia recorded more than 700 new infections and had its deadliest day on Thursday, with 13 deaths.
In the United States, there have been more than 4.4 million infections total. The death count has hit 150,733 — by far the highest in the world, according to a tally by Johns Hopkins University.
The grim milestone comes amid signs that the nation's outbreak is beginning to stabilize in 15 Southern states but rising in the Midwest, brought on by young people who have been maskless and going to bars, restaurants and gyms.