Hundreds of thousands of people in China are now in Covid lockdown as the country battled its worst outbreak in months.

A cluster of infections in Nanjing city, linked to airport workers who cleaned a plane from Russia earlier this month, had reached the capital Beijing and five provinces.

Hundreds of thousands of people have been locked down in Jiangsu province, of which Nanjing is the capital, while 41,000 came under stay-at-home orders in Beijing's Changping district.

At least 206 infections across China have been linked to the cluster, and the outbreak is geographically the largest in several months.

It challenges Beijing's aggressive containment efforts which have relied on mass testing, lockdowns and swift contact tracing.

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The Delta variant is more transmissible than the pathogens that cause SARS, Ebola and smallpox, and as easily spread as chickenpox, according to an internal US Centers for Disease Control presentation reported by The Washington Post and The New York Times.

It has driven recent surges around the world, especially in the Asia-Pacific region.