A zero-tolerance strategy in combating Covid-19 requires strict oversight and ironclad preventive measures. Even the slightest laxness can lead to outbreaks, as mainland China’s present challenges prove. The nation is experiencing its most serious flare-up of the coronavirus in 21 months with scores of daily cases centred on the latest hotspot, the central city of Xian. With the threat of the Delta variant and seemingly more transmissible Omicron strain, increased movement of people for the approaching Lunar New Year holiday and Beijing Winter Olympic Games, there can be no complacency. Xian, with 13 million people, has recorded hundreds of infections since December 9 . A fourth round of citywide mass testing was carried out yesterday and restrictions are the strictest anywhere in the country since a lockdown on Wuhan in Hubei province early last year. Authorities have described the situation as “dire and complex” and warned it will take time to bring the crisis under control. The restrictions disrupt daily life and the economy, but are a necessity to stop the spread of infections. Countries that had decided to live with the coronavirus rather than shut it out have found that the Delta variant is such a threat to public health that many have reimposed restrictions over the Christmas and new year period. At least 8,000 international flights have so far been cancelled this festive season. But Omicron poses an even greater risk; while initial studies appear to show the symptoms are not as severe, the transmissibility is far greater with a doubling of cases every two to three days. China’s zero-Covid aim has zero chance now virus has adapted, Sars expert says The inactivated vaccines mostly used in China’s immunisation programme offer a weak antibody response to Omicron, according to research that has yet to be peer reviewed. That is reason for places that use such jabs, Hong Kong among them, to further tighten border controls and ensure loopholes are closed. The Xian outbreak has been blamed on safety measures not being properly implemented, a slow response to the initial outbreak and prevention and control policies not having been strengthened. Health authorities in Hong Kong are preparing for the worst and putting hundreds more isolation beds on standby in case of a marked upsurge. Hong Kong’s plans for a reopening of the border with the mainland have been put on hold by the Omicron threat and cross-boundary outbreaks. The raising of mandatory quarantine periods to 21 days and stepping up the number of tests for overseas arrivals from any country where Omicron has been detected is the minimum required given the uncertainty. But with more people travelling, what is already in place needs to be reviewed and strengthened. The outbreaks on the mainland show the danger of slackness.