China is dealing with its biggest challenge since Covid-19 first emerged in late 2019. The delta variant was first detected in Nanjing city in mid-July. It is now spreading to places that had been virus-free for months, including the original epicenter of Wuhan. Delta has broken through the country’s virus defenses, which are some of the strictest in the world, and reached nearly half of China’s 32 provinces in just two weeks. While the overall number of infections -- more than 300 so far -- is still lower than Covid resurgences elsewhere, the wide spread indicates that the variant is moving quickly. It’s the biggest challenge to China’s strategy since the virus was first detected in Wuhan, the central Chinese city that saw the world’s first lethal outbreak. The country’s strict anti-virus measures, which include mass testing as soon as a case appears, aggressive contact tracing, widespread use of quarantines and targeted lockdowns, have crushed more than 30 previous flareups over the past year. The arrival of the more infectious delta variant, however, is testing even that approach. The new strain may be exploiting an easing off in masking and social distancing in some places, since much of the country has been Covid-free for months. That, along with increased travel for summer vacation, created an environment where delta could gain a foothold. The broad spread is even more concerning, given the rise of cases in the highly protected capital and in Wuhan, whose virus-free status has been a source of pride in China. The seven new cases there are the first since China brought its original wave under control by locking down the city of some 11 million and the surrounding Hubei province. Pang Xinghuo, the deputy director of Beijing’s Center for Disease Prevention and Control, pleaded with visitors to the city who have traveled from high-risk areas within China or suspect that they might have been in close contact with someone infected to report to authorities. Officials also urged residents to curb their travel and remain in the city for the near future if possible. The initial delta infection arrived via an overseas flight from Moscow into the eastern Chinese city of Nanjing ,and spread on to a group of airport cleaning staff. Within weeks, cases have also popped up as far away as Hainan island in China’s south, 1,900 kilometers (1,180 miles) from Nanjing. China’s vaccination rate is close to 60% and among the highest in the world, but it remains to be seen whether the country’s locally-developed shots can slow delta’s spread. Most of those infected in Nanjing were immunized, and the vaccines -- which have been found less effective than Messenger RNA shots in clinical trials -- do appear to be providing protection, with only 4% of those infected in this current wave battling severe disease so far. Many of those have pre-existing conditions such as asthma, diabetes or high blood pressure, Guo Yanhong, an official with the National Health Commission, said at a briefing in Beijing. While all Covid vaccines are proving less effective against delta, concerns are high that non-mRNA vaccines like the Chinese ones and AstraZeneca Plc’s shot will be less able to slow transmission. State-owned Sinopharm said its inactivated Covid-19 shot, given widely in China, is 68% effective against delta, citing a study in Sri Lanka. Sinovac Biotech Ltd., the other major Chinese supplier, said the antibodies induced by its inactivated Covid vaccine can still neutralize the delta strain in laboratory studies, the state-run Global Times reported, without providing more detail. Delta is providing a reality check for the world, especially countries that thought they were emerging from the pandemic through virus containment or high vaccination levels. Subscribe to our YouTube channel: https://bit.ly/2TwO8Gm Subscribe to our newest channel Quicktake Explained: https://bit.ly/3iERrup Bloomberg Quicktake brings you live global news and original shows spanning business, technology, politics and culture. Make sense of the stories changing your business and your world. To watch complete coverage on Bloomberg Quicktake 24/7, visit http://www.bloomberg.com/qt/live, or watch on Apple TV, Roku, Samsung Smart TV, Fire TV and Android TV on the Bloomberg app. Have a story to tell? Fill out this survey for a chance to have it featured on Bloomberg Quicktake: https://cor.us/surveys/27AF30 Connect with us on… YouTube: https://www.youtube.com/user/Bloomberg Breaking News on YouTube: https://www.youtube.com/c/BloombergQuickTakeNews Twitter: https://twitter.com/quicktake Facebook: https://www.facebook.com/quicktake Instagram: https://www.instagram.com/quicktake