Fears about the Delta variant of the coronavirus sparking a new global wave have proven accurate, and it is rapidly becoming the dominant strain of the disease. Mainland China, despite its admirable record of warding off infections, is now battling the mutation, with hundreds of cases in Nanjing and an increasing roster of towns and cities in at least seven provinces. Prevention and control measures that have been working so well have developed weaknesses and authorities have called for their strengthening. Beating the outbreak requires all people making an effort; it is a strategy that has served the nation well before and, with the same determination, it can again succeed. The outbreak stemmed from a cleaner at Nanjing’s international airport being infected while working on a cargo plane that had arrived from Russia on July 10. Fellow cleaners and other staff and passengers subsequently tested positive. Outbreaks centre on Nanjing, the capital of the eastern province of Jiangsu, and the tourist city of Zhangjiajie in the central province of Hunan. But the virus is spreading, apparently the result of tracing systems not being as robust as before due to the nature of the Delta strain. In an effort to stop the disease from taking hold in Beijing, transport from affected areas has been banned. Authorities are understandably worried; although there have been clusters of infections from time to time in various parts of the mainland, this is the worst outbreak in more than five months. Vice-Premier Sun Chunlan, who so successfully oversaw the eradication of the coronavirus from Wuhan, the first city in which it appeared in China, visited Nanjing for four days until Sunday to inspect treatment and testing facilities and urge tougher action. She said preventing more cases from being brought in from outside was the most important control measure and that agencies “must not let their guard down, not even for one minute”. China reports 71 local Covid-19 cases as pressure builds on officials It is essential advice that has been ignored, as the outbreak proves. Fuelling its spread has been lax prevention measures at Nanjing’s airport, an infected elderly woman who visited several mahjong parlours in the neighbouring city of Yangzhou, and a performance gala at a theatre in Zhangjiajie at which 2,000 people not wearing masks sat side by side. After almost 19 months of social distancing, mandatory testing and lockdowns, Covid-19 fatigue is to be expected. But given the world is still struggling with the virus, vigilance has to remain a priority. Vaccination and herd immunity hold the key and the mainland is heading towards that goal. But the Delta variant lessens the efficacy of vaccines, which wear off over time, necessitating booster jabs. In the meantime, testing and isolating, improved tracing, social distancing and mask-wearing are essential.