The city where Covid-19 was first detected reported new infections for the first time in more than a year, prompting authorities there to launch mass testing, as China fights to contain several outbreaks of the more infectious Delta variant.

Health officials in the central Chinese city of Wuhan ordered the testing on Tuesday after seven new cases were detected there on Monday. The city of 11 million people, which went into one of the world’s strictest lockdowns for two and a half months, hasn’t reported any local infections since June 2020.

Meanwhile, the top municipal official in Beijing cut off travel into the capital to protect it from exposure to the virus—part of the tightest pandemic control measures China has implemented since February, Goldman Sachs said in a note.

China on Tuesday logged 90 new confirmed cases, 61 of which were locally transmitted, according to China’s National Health Commission. That brings the total number of new cases to roughly 400 in more than a dozen provinces since the discovery in mid-July of a Delta cluster in the eastern city of Nanjing.

Chinese health authorities distinguish between imported and locally transmitted infections, and only include symptomatic cases into the official tally. Local health agencies occasionally list asymptomatic cases separately.

People line up for Covid-19 testing in front of Hankou Water Tower in Wuhan.

People line up for Covid-19 testing in front of Hankou Water Tower in Wuhan.

Photo: china daily/Reuters

With 1.69 billion shots of Covid-19 vaccines administered in China’s population of 1.41 billion people by the end of Monday, according to statistics by the National Health Commission, much of the transmission is silent. Patients often don’t have any or only very mild symptoms and therefore don’t seek medical attention.

The continuing spread of the Delta variant in China, though modest by global standards, poses a challenge to the government, which has strictly controlled borders and instituted localized lockdowns for more than a year in a bid to eradicate the virus entirely inside the country.

The first nine cases amid airport workers in the Nanjing outbreak, which public health experts traced to a passenger on an aircraft that arrived in the city from Russia on July 10, were only discovered 10 days later during a routine mass testing. Meanwhile, the virus spread from Nanjing, a domestic transportation hub, all across China.

Nanjing, the capital of Jiangsu province, launched the fourth round of the citywide mass testing of its 9.3 million residents in two weeks, according to a statement released by local health authorities on Monday. Half of the detected cases were found in quarantine centers for close contacts and nearly 30% tested positive during the screening of key personnel and communities.

Jiangsu, the province with the second-largest economic output in China after Guangdong, accounts for about 80% of the recently confirmed cases.

To contain the flare-ups, local officials in affected areas have reinstated stricter pandemic measures and urged residents to avoid nonessential travel.

On Sunday, Beijing’s Communist Party Secretary Cai Qi said that the capital had to be protected “at any cost,” suspending planes and trains from places with Covid-19 flare ups from entering the city.

Residents as well as visitors in Zhangjiajie, a tourist hot spot in southern China and the source of a second superspreader event involving a theater performance attended by more than 2,000 guests on July 22, were barred from leaving the city starting Tuesday.

Nanjing and the nearby city of Yangzhou to its northeast have also restricted passenger road traffic and suspended all domestic flights and online car hailing services.

Confirmed Covid-19 cases in Yangzhou have shot up to 94 from an initial two discovered less than a week ago. The source of the outbreak in that city was a 64-year old woman who had become infected in Nanjing and subsequently visited three local mahjong parlors popular with older people, according to local health authorities.

On Tuesday, China’s Ministry of Education issued a notice urging local authorities across the country to promote student vaccination. In June, China approved the emergency use of the Sinovac Covid-19 vaccine for children as young as three.

An airport worker in Shanghai also tested positive on Monday, according to local health officials. His case is believed to be separate from the Nanjing cluster.

Write to Sha Hua at sha.hua@wsj.com