The team behind a Chinese Covid-19 vaccine candidate say early phase trials of their product suggest it is unlikely to cause a potentially harmful antibody-dependent enhancement (ADE) of the disease.
ADE is a concern for vaccine developers because the mechanisms that underlie antibody protection against a virus can also amplify the infection or trigger harmful immune responses.
Scientists at the Institute of Medical Biology, under the Chinese Academy of Medical Sciences, conducted phase 1 trials of their product on 191 people aged 18 to 59 in May. All of the subjects were from southwest China’s Sichuan province and had never been infected with the coronavirus.
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According to the team’s findings – which were published on Tuesday on the preprint website medRxiv.org and have not been peer-reviewed – in the first 28 days after vaccination, none of the subjects experienced a severe adverse reaction.
The most common effects were mild pain, slight fatigue and redness at the point of injection, they said.
“Although we cannot conclude that this vaccine will not cause ADE, these observations at least suggest that the likelihood of ADE as a result of inoculation with this vaccine is small,” the researchers said.
“All the data obtained in this trial support the safety and immunogenicity [the ability to provoke an immune response] of this inactivated vaccine and are encouraging with regards to further studies of its efficacy.”
The team began phase 2 trials of the product – involving 750 adults from southwest China’s Yunnan province – in June, and tests involving an over-60s group were ongoing, Ying Zhang, who co-authored the study, said in an interview.
“Statistics from both trials of nearly 1,000 people show the safety of the vaccine,” she said. “We want to make sure the vaccine is not only effective but also will not cause hidden side effects, such as reproductive toxicity which might only be revealed later.”
Final, phase 3, trials of the vaccine candidate, involving an estimated 25,000 to 30,000 people, would take place outside China, where the coronavirus was more active, though the exact location had not yet been decided, Zhang said.
According to Professor Jin Dong-yan, a molecular virologist at the Li Ka Shing Faculty of Medicine at the University of Hong Kong, the risk with new vaccines was their potential to trigger ADE, which effectively made the virus more harmful than it was before inoculation.
If a vaccine produced such a reaction, it was not fit for use and its developers would have to identify and remove the part that was triggering ADE, Jin said.
Zhang said she and her team tested for an ADE response by incubating diluted immune serum samples from vaccinated people with the coronavirus to observe whether infection was magnified as the concentration of antibodies dropped.
“The scientists took a step further than other studies [to test for ADE in vitro],” Jin said. “There’s merit in their work but it still does not provide a definite answer,” he said.
“What might actually happen when these people are infected by Sars-CoV-2 [the formal name for the virus that causes Covid-19] during different phases is still to be determined.”
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This article Coronavirus: risk of ADE low with new Chinese vaccine candidate, researchers say first appeared on South China Morning Post