China is confronting its broadest COVID-19 outbreak since coronavirus first emerged there in late 2019, with the delta variant 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 resurgences elsewhere, the wide spread indicates that the variant is moving quickly.

The resurgence marks 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.