HONG KONG—China’s coronavirus controls, which have largely succeeded in keeping Covid-19 at bay over the past year, are facing a fresh test as the highly infectious Delta variant spreads quickly within the country.

The cluster of cases, which has infected at least 200 people, is modest by global standards. But infections have spread quickly to at least 26 cities in 16 provinces since the cluster was first detected on July 20 during a routine test of nine airport cleaners in Nanjing, a city of 9.3 million people in eastern China.

The rapid spread of infections by people with no symptoms—or only very mild ones—in the latest cluster poses a challenge to China, which has strictly controlled its borders for more than a year while implementing a regimen of mass testing and quarantine domestically. Many of the cases have remained undetected for days before they were found through contact tracing or during mass testing.

While authorities have administered 1.62 billion vaccine doses through Thursday, infections among fully vaccinated people have triggered doubts over the protection currently offered by Chinese vaccines against the Delta variant, and accelerated calls for booster shots.

A woman in Nanjing gets tested for Covid-19 a day after the local outbreak was detected.

A woman in Nanjing gets tested for Covid-19 a day after the local outbreak was detected.

Photo: str/Agence France-Presse/Getty Images

At the same time, more than a year with only sporadic and locally contained outbreaks has worn down public vigilance. In many parts of the country, mask-wearing and checking of digital health codes—once unavoidable—have become an afterthought.

The new surge provides “food for thought” for China’s future pandemic strategy, Zhang Wenhong, a prominent public-health expert, wrote on China’s Twitter -like Weibo platform on Thursday.

Mr. Zhang, director of the infectious diseases department at Shanghai’s Huashan Hospital, raised the question of whether China needs to adjust its strategy to better coexist with the virus and its risks, rather than to try to completely shut it out and eradicate it.

A theater performance attended by more than 2,000 people in Zhangjiajie, a tourist hot spot in the southern province of Hunan, has become the focus of concern as a potential second superspreader cluster. At least two cases in Beijing, the capital, have been linked to the Zhangjiajie cluster.

Four infected cases from the northeastern port city of Dalian had transited at Nanjing’s airport on the way to the theater performance in Zhangjiajie.

Last week, Nanjing public-health officials told local media that most of the cases were breakthrough infections—cases in which someone contracts Covid-19 more than 14 days after they have been fully vaccinated.

Zhong Nanshan, the country’s best-known epidemiologist, told state broadcaster China Central Television in May that China needed to vaccinate 80% of its population to achieve herd immunity—a goal that Goldman Sachs economists estimate may not be attainable until the end of the year.

Other prominent Chinese public-health experts have said in recent weeks that booster shots would likely be needed, pending trial results, given the current resurgence and new variants.

People line up for Covid-19 testing at a sports center in the Gulou district of Nanjing.

People line up for Covid-19 testing at a sports center in the Gulou district of Nanjing.

Photo: stringer/Reuters

While antibodies for most recipients of Sinovac Biotech Ltd.’s CoronaVac declined below a key threshold six to eight months after the second dose, a third shot administered at that time had a strong booster effect, Chinese researchers wrote in a study published Sunday. The findings, which weren’t peer-reviewed, echoed comments made by Sinovac’s chief executive last month.

Health officials in Western countries are also studying whether booster shots are needed, and if so, for whom and under what circumstances.

Governments across the world have been struggling with the question of when and how to reopen their economies while the Delta variant spreads.

While the United Kingdom chose to drop almost all coronavirus-related restrictions on July 19, countries pursuing a “zero-Covid” strategy, including China and Australia, have said they plan to keep strict border controls in place into 2022.

Mr. Zhang, the public health expert at Shanghai’s Huashan Hospital, argues that even if everyone in China were to be vaccinated, Covid-19 would still exist and could become an endemic disease, similar to influenza, with a lower death rate.

China has strictly controlled its borders for more than a year while implementing a regimen of mass testing and quarantine domestically.

China has strictly controlled its borders for more than a year while implementing a regimen of mass testing and quarantine domestically.

Photo: Ng Han Guan/Associated Press

Pointing to data from the U.K. and Israel, where the proportion of fully vaccinated citizens is close to 70%, Mr. Zhang noted that hospitalizations and deaths have remained modest even amid the surge in new infections.

In June, Feng Zijian, former deputy head of the Chinese Center for Disease Control and Prevention, told a conference that the timing of any shift from a zero-Covid-19 strategy to one with more open borders would largely depend on a high vaccination rate, though he ultimately said any call would require a social and political consensus on the trade-offs.

China’s infection numbers have remained low since March 2020, and local outbreaks were quickly contained with a combination of citywide mass testing, targeted residential lockdowns, temporary travel restrictions, contact tracing and quarantining of close contacts.

In response to the most recent outbreak, Nanjing authorities have closed cinemas, gyms and bars and urged people to cancel nonessential travel. Airport personnel will be tested every three days rather than once a week.

In southern China, the large cities of Guangzhou and Shenzhen plan to funnel future inbound travelers to specially built quarantine centers designed to minimize human-to-human contact amid the spread of the Delta variant.

At least two cases in Beijing have been linked to a possible superspreader event in another city.

At least two cases in Beijing have been linked to a possible superspreader event in another city.

Photo: roman pilipey/Shutterstock

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