Four minutes and 52 seconds. That’s the time it took Bridget Mutambirwa to take a nucleic acid test for the novel coronavirus disease (COVID-19) in Beijing on July 3. Just 24 hours later, the result was released on the Health Kit applet. It was negative. “The whole process was faster than I expected,” the 37-year-old Zimbabwean told Beijing Review.

Three days later, Beijing reported no new domestically transmitted cases, the first time after new cases kept surfacing in the capital since June 11. From June 11 to July 5, the city reported 335 locally transmitted cases, of whom 47 percent worked in the Xinfadi wholesale market in Fengtai District in southwest Beijing, according to the Beijing Municipality Government. It took Beijing 26 days to bring the number to zero.

“Beijing didn’t become the second Wuhan or New York thanks to fast, well-targeted and draconian epidemic prevention and control measures,” Wu Zunyou, chief epidemiologist with the Chinese Center for Disease Control and Prevention, said in an video interview with The Beijing News on July 3.

After the first new case was reported on June 11, it took only 22 hours to locate the hot spot—the Xinfadi wholesale market, the largest of its kind in the capital. The market was locked down within 48 hours. “The timely lockdown of Xinfadi cut off the main transmission channel,” Wu said.

All close contacts of the new case and more than 8,000 business people working in the market underwent the nucleic acid test within a day of the closure and were put under medical observation. Epidemiologists found that many of the Xinfadi people with a positive test result had no symptoms in mid-June. “It means they were tested and detected at an early stage of the disease,” Wu said.

Among all the 335 new cases in Beijing, 98 percent had mild symptoms. Wu thinks two factors contributed to this. “Many patients are young and got timely medical care before showing any symptoms,” he said.

Mass nucleic acid tests are a good way to identify asymptomatic cases. Beijing built temporary sample collecting points in communities and inflatable laboratories and implemented 24-hour testing in shifts and batches, thereby increasing its daily testing capability by five times, from 100,000 samples in mid-June to 500,000 in early July.