A domestically produced inactivated COVID-19 vaccine displayed at the China International Fair for Trade in Services held in Beijing from September 4 to 9. Photo: Li Hao/GT

The antibody-dependent enhancement (ADE) effect, which could lead to the failure of vaccines and infections that are even more serious, has not yet been observed from clinical trials of China-developed COVID-19 vaccines, said Li Yingli, the chief reviewer in charge of clinical review of vaccines at the Center for Drug Evaluation of the Chinese National Medical Products Administration, on Saturday.

"ADE is a problem that we are particularly concerned about. If it occurs in subjects after vaccination, the vaccine must be given up," Li told the Global Times at the First Greater Bay Area Vaccine Summit held in Shenzhen, Guangdong Province.

ADE refers to a phenomenon in which the mechanisms that provide antibody protection against any virus theoretically have the potential to amplify the infection or trigger harmful immunopathology. This means the immune response will possibly cause the disease to worsen instead of protecting the subject, and has sparked widespread concerns over the efficacy and safety of COVID-19 vaccines.

Gao Fu, the head of the Chinese Center for Disease Control and Prevention, said at the event that a vaccine is crucial for the population to achieve herd immunity against COVID-19, and ADE is one of the biggest challenges that vaccine development is now confronted with.

ADE has been observed in many viruses, especially the notorious dengue disease and feline infectious peritonitis, a progressive and fatal disease caused by the feline coronavirus that is closely related to novel coronavirus, which led to people's concerns, Gao told the Global Times.

However, whether ADE will occur or not for novel coronavirus has not yet been determined, and there is no data for reference at home or abroad on this question, Gao noted. 

"We should still stay alert to ADE in vaccine development," he said.

Widespread concerns over the efficacy of COVID-19 vaccines were raised after global trials of a coronavirus vaccine co-developed by AstraZeneca and Oxford University were suspended recently when a volunteer fell ill during trials. Concerns were raised in the wake of second infection cases reported in places including Hong Kong, Brazil and Europe. 

"These are all individual cases," Gao said, stressing that such individual cases should not be over-interpreted and calling for a cautious and optimistic attitude to a COVID-19 vaccine.

Meanwhile, Chinese experts said that research should be conducted on reinfection cases. 

"By studying such cases, we can find out what kind of immunity is the most important and essential against the novel coronavirus," Dong Chen, director of the Institute for Immunology of Tsinghua University, said on Saturday.

Dong warned of a possibility of widespread re-infection and a resurgence of novel coronavirus in winter, especially in overseas regions, as the season sees high incidences of respiratory diseases and the vaccine is not on the market yet.

"I'm confident that we will not experience the outbreak that happened in Wuhan again, but there is still a possibility that a resurgence like the one in Beijing or one that is more serious, so everybody should still keep alert to the pandemic," Gao said, noting that he is also confident that China will bring a large-scale outbreak under control if one does occur, considering the previous experience gained from the outbreaks in Wuhan, Beijing, Xinjiang and northeastern Chinese provinces.

Experts advised people to wait patiently for vaccination until the third phase of clinical trial is completed and its efficacy can be fully proved, although priority groups in China, such as medical staff, diplomats sent to highly affected countries, and employees sent overseas under the Belt and Road Initiative, are receiving vaccination for emergency use.