- Officials in Nanjing are now on "high alert" and plan to test all the city's residents, having already got through 1.9 million people in one day, the Xinhua news agency reported Thursday.
For most countries, the spike in Covid-19 cases playing out in China would register as little more than a blip on the chart.
But the 184 infections detected in the city of Nanjing in the past 10 days will test whether the country's zero-tolerance approach, which has swiftly dealt with previous outbreaks, can contain the highly transmissible delta variant.
The outbreak is believed to have started in Nanjing, a city of more than 9 million people, around 150 miles west of Shanghai.
On July 20, nine cleaners at the Nanjing Lukou International Airport tested positive for the virus after cleaning down a plane that arrived from Russia, according to state-run media. Infections have now been recorded in five different provinces including Beijing, and local health authorities have identified the strain as the delta variant.
Officials in Nanjing are now on "high alert" and plan to test all the city's residents, having already got through 1.9 million people in one day, the Xinhua news agency reported Thursday.
China has crushed previous spikes with strictly enforced lockdowns, quarantines and controls on international travel.
After the initial Covid outbreak, which was first recorded in Wuhan in December 2019, China's flare-ups have been relatively minor by international standards and in the context of China's 1.4 billion population.
The United States had 202.4 infections for every million people Thursday, according to Johns Hopkins University. The same figure for China was 0.04.
But daily cases in China are now at their highest since January, when tens of millions of people were placed under lockdowns as authorities battled to suppress cases centered around Hebei province.
The current outbreak has drawn a strong rebuke from the Central Commission for Discipline Inspection, a senior disciplinary arm of the Chinese Communist Party. It said the Nanjing airport did not separate cleaners for international and domestic flights, and criticized the airport's management for what it said was a lax approach at preventing further spread.
Most recent outbreaks have been in smaller cities. But Nanjing is a major population center, among the country's top ten cities that has significance in sectors such as science and education.
The rise in cases has also led to questions about the effectiveness of China's Covid vaccines, made by the state-owned company Sinopharm and the private firm Sinovac.
Other countries that have used these Chinese vaccines have experienced waves of infections. More than 30 Indonesian health workers died despite receiving two doses of Sinovac.
And Chile reintroduced restrictions after a wave of the delta variant ripped through its population, 70 percent of whom have been fully vaccinated mostly with Sinovac.
Some experts say other factors may have been at play, including people mixing too quickly following their first dose in Chile. Others believe the Chinese vaccines have shown good protection against severe disease.
And it is possible, although rare, for people to catch Covid and become seriously ill after two doses of any vaccine.
China has not released results of clinical trials or real-world data that would be needed to assess the vaccines' effectiveness against different variants. This makes peer-reviewed analysis difficult.
Both Sinopharm and Sinovac say their shots provide sufficient protection against delta. And people in China who have been fully vaccinated have mostly experienced mild symptoms during the current crop of infections, according to state media.
The rest of the pandemic-weary world has watched as life appeared to return to normal for many Chinese people, with images of crowded pool parties and bustling theme parks contrasting sharply with lockdowns in the West.
The country and its government will now be hoping its current outbreaks don't turn back the clock to the dark days of early 2020.