Delta variant challenges China's zero Covid strategy — and raises questions over its vaccine efficacy

Citizens line up at a Covid-19 testing site in Yuhuatai district of Nanjing, in eastern China's Jiangsu province, on August 2, 2021.

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Hong Kong (CNN)China's hardline zero Covid strategy is facing a fresh challenge from the rapid spread of the Delta variant, amid concerns over the efficacy of Chinese vaccines against the highly contagious strain.

The Delta variant, which appears to cause more severe illness and spreads as easily as chickenpox according to an internal document from the US Centers for Disease Control and Prevention (CDC), has wreaked havoc across the world. Now, it is causing China's worst outbreak in months.
China reported 328 local Covid-19 infections in July, close to the total from the previous five months, according to the National Health Commission. Although that's only a fraction of the cases reported in countries like the United States and the United Kingdom, it is perceived as a serious threat in China, where authorities attempt to keep local infections at zero.
    The latest outbreak started two weeks ago in the eastern city of Nanjing, where nine airport cleaners were found to be infected during a routine test. Since then, the cluster has spread to at least 26 cities across China, including a tourist hot spot in the southern province of Hunan and the capital Beijing.
      Chinese authorities responded swiftly with mass testing, targeted lockdowns, extensive contact tracing and quarantine of close contacts — a tried and tested formula that has helped it quickly tame local flare-ups since March 2020.
      In Nanjing, a city of 9.3 million people, residents went through three rounds of coronavirus testing in two weeks. Residential compounds with confirmed cases were locked down, while cinemas, gyms, bars and libraries were shuttered.
      In Zhangjiajie, where a theater performance attended by thousands of people has fueled concerns of a super-spreader event linked to the Nanjing cluster, all tourist destinations were shut down and residents are banned from leaving their compounds.
        Restrictions have also been ramped up in Beijing, which reported its first local infection in nearly six months last week. Since then, five local cases with travel history to Zhangjiajie have been detected. The city has banned people from medium- or high-risk areas from entering, suspending flights, trains and buses from Covid-hit places.
        Huang Yanzhong, senior fellow for Global Health at the Council on Foreign Relations, said Chinese authorities are responding to the threat of the Delta variant with the same playbook used in previous outbreaks, but in an even more stringent manner — several cities have been placed under effective lockdowns.
        "It shows the problem of diminishing returns of the existing 'zero-tolerance' approach," he said.
        Elsewhere in the world, countries with relatively high vaccination rates are increasingly choosing to tolerate a degree of transmission, as long as it doesn't translate into a surge in hospitalizations and deaths. In China, however, authorities appear intent on imposing strict restrictions despite having administered more than 1.65 billion homegrown vaccine doses — ranking above the US in terms of doses administered per 100 people.
        The rapid spread of the Nanjing cluster to cities across the country has triggered concerns over the level of protection currently offered by Chinese vaccines against the Delta variant.
        On July 22, two days after the Nanjing cluster was first detected, a health expert in the city said the "vast majority" of those infected there had been vaccinated, except for one person under 18 years old. Airport staff, along with medical workers and border control personnel, were among the first to be vaccinated in China.
        Chinese authorities have not offered a fuller breakdown of how many subsequent cases were also vaccinated.
        Several other countries that used Chinese vaccines have also reported infections among vaccinated people. In June, Indonesian officials said more than 350 medical workers in the country had contracted Covid-19 despite being inoculated with vaccines produced by Chinese company Sinovac Biotech.