A Chinese city on the border with Vietnam has put 3.6 million residents under strict lockdown after finding dozens of infections during mass testing. Baise, in the southern region of Guangxi, reported one Covid-19 infection on Saturday in a resident who had recently returned to the city. Mass testing was then conducted on more than 207,000 residents in Debao county, where the Covid-positive resident lives. Ninety-eight people were found to be positive and the results of a further 87 had to be re-examined, the municipal government said on Sunday. The city reported 37 confirmed cases – cases in which the infected patient has symptoms – on Sunday. Huang Zhengyong, deputy director of Guangxi Centre for Disease Control and Prevention, said all the cases were in the same transmission chain and the first two cases were confirmed infected with the Omicron variant of the coronavirus. “The genome sequencing comparison is still ongoing but the index case is very likely to have been infected outside Guangxi,” Huang said. “The first case attended numerous gatherings and those present at the meals accounted for half of the current infections. The virus was then transmitted further in families and communities.” Across China, 45 local Covid-19 cases were reported on Sunday, a rise from the low of nine cases last week during the Lunar New Year holiday. Active cases have been identified in Beijing, Guangzhou, Tianjin, Hangzhou and Shenzhen amid China’s determination to achieve a “dynamic zero-Covid” – to quickly cut each chain of transmission and prevent further spread in the community with quick testing and interventions such as travel restrictions and quarantine. The country’s Covid-19 response is being tested during the Winter Olympics, which started on Friday, with participants allowed to enter the “closed loop” of the games without quarantine if they are fully vaccinated. By Sunday, 154 athletes and team officials and 233 other participants had tested positive since January 23. There has been no report of Covid-19 spreading outside the loop yet. Debao county announced it would start another round of testing on Monday “to strengthen early prevention and rapid interruption of virus transmission in view of the current severe epidemic situation”, according to the county government. “Those who refuse to cooperate, hinder the prevention and control of the epidemic or cause other serious consequences without a valid reason will have to bear the corresponding responsibility according to law,” the authority said. “Anybody who misses the testing once will be given a yellow health code; missing testing twice will result in a red health code,” it said, referring to the health code system that only allows green health code carriers to travel or be admitted to public venues. Baise imposed travel restrictions on Sunday forbidding residents and vehicles from moving between counties or districts. No residents are allowed to leave or enter the city. On Monday, the restrictions were upgraded to home quarantine, applying to all residents until further notice. Omicron may make China’s Covid success ‘fleeting’, says US report All schools and training institutions suspended their classes and public transport services were halted. All business venues will be shut down except supermarkets and grocery markets, hospitals and pharmacies. Workers deemed essential – such as food deliverers and those engaged in epidemic control and medical services – can travel within the city with permits. “Businesses that open without authorisation will be shut down and those who leave home or gather without authorisation will be punished,” the Baise government said in a statement, adding that the public should fulfil their legal obligations and cooperate with and support virus prevention and control measures. Guangxi Communist Party secretary Liu Ning, said on Sunday the situation in Baise was “very serious” and prevention and control of the epidemic must not be relaxed “for one moment”. “The string of epidemic prevention and control should always be tightened and [we will] resolutely win the battle of annihilating Covid-19 with higher political consciousness, stronger responsibility, stricter prevention and control measures and more realistic work,” Liu said.