A Chinese health expert helping oversee coronavirus control efforts in a northeastern Chinese city at the centre of an outbreak has declared the emergency over after mass testing revealed a handful of asymptomatic cases. Authorities in Mudanjiang, Heilongjiang province, near the border with Russia tested more than 650,000 people in the city in the last week, becoming the second municipality in the country to test all of its residents for the coronavirus . The city battled an influx of cases as Chinese nationals returned from Russia in April. Mudanjiang mayor Wang Wenli said on Monday that only 19 asymptomatic cases were found in the thousands of nucleic acid tests. The symptom-free carriers were aged between 5 and 70 and a total of 1,529 close contacts were traced, Wang said. Sun Dianjun, director of a provincial panel overseeing the coronavirus control efforts, said: “Based on the nucleic acid testing results, I think the epidemic in Mudanjiang has stopped.” Mudanjiang’s mass testing programme came less than a month after the central Chinese city of Wuhan, the initial epicentre of the pandemic, carried out similar tests on nearly 10 million of its residents. The Wuhan Health Commission said on Monday that viral cultures of the samples taken from 300 asymptomatic carriers detected in the mass tests were negative, indicating that the carriers had a very low amount of the virus or there was no “live virus” that could make others sick. The samples from the asymptomatic carriers were analysed at the Wuhan Institute of Virology. The commission said the asymptomatic cases in Wuhan were not infectious, since their 1,174 close contacts all tested negative. “The viral culture tests were conducted to make the epidemic control work more precise, to manage the symptom-free carriers in a more scientific way and to let the whole country including the public of Wuhan feel more relieved,” the commission said. Zhang Dingyu, vice director of the Hubei Health Commission, told the Yangtze Daily , that viral culture laboratory test was a common method used to check the capacity of a virus to cause disease. “We did this on the 300 asymptomatic cases so that we can know if there is ‘live virus’ in their bodies and what the virus amount is,” Zhang said. “That way we can understand the infectiousness of the cases and can manage them accordingly.” Yu Xuejie, dean of the Wuhan University school of health sciences, said the low infectiousness of asymptomatic carriers in Wuhan was also reflected in the small number of new cases in the city over the past three months. “But we cannot conclude that it’s the same situation in other countries,” Yu said. “That’s because the coronavirus might mutate and the virus in other parts of the world might be not the same strain as that in China right now.” A World Health Organisation official said on Monday that transmission from symptom-free cases was “very rare”. “It still appears to be rare that an asymptomatic individual actually transmits [Sars-CoV-2] onward to a secondary individual,” Maria Van Kerkhove, the WHO’s Covid-19 technical lead, said. China’s State Council has also notified local governments on how to “normalise” Covid-19 control work. Local authorities are required to expand nucleic acid testing capacity to ensure tests on eight categories of people, including travellers from overseas, medical staff, border officers and nursing home staff.