Some scientists also worry an Ad5-based vaccine could increase chances of contracting HIV
Toronto/Chicago — High-profile Covid-19 vaccines developed in Russia and China share a potential shortcoming: they are based on a common cold virus that many people have been exposed to, potentially limiting their effectiveness, some experts say.
“The Ad5 concerns me just because a lot of people have immunity,” said Anna Durbin, a vaccine researcher at Johns Hopkins University. “I'm not sure what their strategy is ... maybe it won't have 70% efficacy. It might have 40% efficacy, and that's better than nothing, until something else comes along.”
Researchers have experimented with Ad5-based vaccines against a variety of infections for decades, but none are widely used. They employ harmless viruses as “vectors” to ferry genes from the target virus — in this case the novel coronavirus — into human cells, prompting an immune response to fight the actual virus.
Dr Zhou Xing, from Canada’s McMaster University, worked with CanSino on its first Ad5-based vaccine, for tuberculosis, in 2011. His team is developing an inhaled Ad5 Covid-19 vaccine, theorising it could circumvent pre-existing immunity issues.Xing also worries that high doses of the Ad5 vector in the CanSino vaccine could induce fever, fuelling vaccine scepticism.
Researchers, including top US infectious diseases expert Dr Anthony Fauci, in a 2015 paper, said the side effect was likely unique to HIV vaccines. But they cautioned that HIV incidence should be monitored during and after trials of all Ad5-based vaccines in at-risk populations.
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