The Delta variants has now invaded China, indicating its high-risk potential even against the most aggressive Covid-19 containment regimes, according to a report put out by Bloomberg.

An outbreak that started at an airport in the eastern Chinese city of Nanjing is testing that country’s zero-tolerance measures, which are some of the most sweeping and comprehensive in the world. New infections are rising by the dozens and seeding subsequent clusters around China despite well-honed systems of mass testing and stringent quarantines. Beijing reported its first locally-transmitted infection in six months on Thursday, linked to an outbreak in the southern province of Hunan among people who had recently been to Nanjing, reports Bloomberg.

The variant is scaling some of the toughest virus defenses, with “Covid Zero” places — countries that had snuffed out the virus within their borders — still seeing outbreaks despite strict anti-virus measures.

Among the hardest hit is Australia, where delta is slipping through the mandatory hotel quarantine system far more easily than past strains and taking advantage of a low community vaccination rate. A delta-fueled outbreak even forced Sydney, despite its efficient contact tracing and testing apparatus, into weeks of lockdown, with cases climbing to nearly 3,000 since mid-June.

In China, the first infections were among nine airport cleaners. The cluster quickly expanded to their close contacts, then to a handful of other locations, leading to nearly 200 confirmed Covid cases as of Thursday. It’s one of China’s biggest outbreaks since a wave concentrated in the country’s northeast saw more than 2,000 infections last winter, reported Bloomberg.

The reports quoted unnamed officials confirming that the new outbreak is caused by the delta strain, which has been driving a resurgence in infections across the world.

According to the report, many of the people infected in China, including the Nanjing airport workers, had been fully vaccinated -- and only four have developed severe cases of the disease -- according to official data. The numbers signal that the immunity generated by China’s vaccines while enough to ward off critical illness and death is still insufficient to prevent the spread of the variants.