“These were all mild cases,” Yang said. “Although they haven’t been ill for long, based on what we’ve observed in the recent outbreaks in Guangdong and Ruili [in Yunnan province], there is a significantly lower chance of [someone who is vaccinated] getting severely ill and the course of the disease is also shorter,” he said.

“So this shows that the vaccines are still protecting people and we urge you to take precautions and continue to wear a mask even after you get vaccinated.”

01:15

China rejects WHO plan to revisit Covid-19 lab leak theory

China rejects WHO plan to revisit Covid-19 lab leak theory

While China is largely free of Covid-19, breakthrough infections have been detected among high-risk groups, especially at airports and hospitals whose workers were among the first to be vaccinated.

At Shenzhen’s international airport last month, a customs worker tested positive for the Delta strain, followed by a restaurant employee. Both of these breakthrough infections were found to be related to cases that came in on a flight from South Africa.

Earlier this week, a nurse in Qingyuan, Guangdong who looked after a Covid-19 patient tested positive for the Alpha variant. She had also been vaccinated.

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In March, a doctor testing for Covid-19 in Xian, Shaanxi became the first known case in the country of the Alpha variant but her symptoms were mild and she recovered within three days.

The doctor’s case – the first known breakthrough infection in a health care worker – was assessed by researchers at the Chinese Centre for Disease Control and Infection.

They noted in the journal China CDC Weekly in May that breakthrough infections happened with all vaccines and needed further research. “We suggest conducting routine, systematic study of every fully vaccinated breakthrough case,” they said. “This should include tracing the source of the virus, identifying the virus lineage to find variants, measuring antibody levels, and assessing for vaccine-associated enhancement of disease.”

On Thursday, Nanjing Health Commission director Fang Zhongyou said the source of the airport cluster had been identified through initial genome sequencing but did not elaborate, though he suggested it appeared to be related to an imported case.

Breakthrough infections happen when vaccines do not trigger enough of an immune response or if the immune system does not produce enough antibodies, according to Ma Wenjun, chief scientist with the Guangdong Centre for Disease Control and Prevention. They can also happen when a virus variant is resistant to a vaccine.

“There is no vaccine that is 100 per cent protective, and breakthrough infections will always happen, more or less. People should have a clear understanding of that,” Ma wrote in an article published by the Guangdong Preventive Medicine Association last month.

He said vaccination still played an important role in preventing large Covid-19 outbreaks, and reducing the severity of cases and deaths.

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Jin Dong-yan, a virologist with the University of Hong Kong, said breakthrough infections pointed to the continued need for protective measures and for “booster injections to be given as soon as possible”.

In Nanjing, mass screening is under way in the city of 9 million people, with more than 5 million tests conducted so far. Its international airport said it had begun testing staff in “key posts” every three days instead of every week. Testing will also be more frequent at Shenzhen’s international airport, with frontline workers to avoid contact with the public where possible.

Additional reporting by Holly Chik