China's top COVID-19 vaccine candidate showed weak results among older Phase II test subjects

China's CanSino Biologics reported preliminary results of its Phase II COVID-19 vaccine trial Monday, and there was promising news. But it was overshadowed by the results of the Oxford University-AstraZeneca vaccine, also published Monday in the journal The Lancet. Both vaccines, among the top contenders in the global race for a coronavirus immunization drug, produced strong immune responses with only minor side effects, but older trial participants showed significantly weaker responses in the CanSino trial, suggesting two doses may be needed.

CanSino's vaccine appears "pretty weak compared to other vaccine candidates (to the extent that comparisons are possible)," said Prof. John Moore at Weill Cornell Medical School. However, comparing the immune response among different vaccines is tricky, he added, "like judging a beautiful baby photo contest when every mom uses a different Instagram filter."

The Oxford-AstraZeneca vaccine and CanSino's candidate both use genetically deactivated adenoviruses that mimic the new coronavirus and stimulate an immune response. But while Oxford's vaccine uses an adenovirus found in chimps, CanSino relies on an adenovirus that causes the common cold in humans. Monday's CanSino study "revealed that people who had previously been exposed to the cold virus showed weaker immune responses to the coronavirus vaccine — presumably because their immune systems zeroed in on the familiar component of the vaccine, the weakened cold virus, rather than SARS-CoV-2," Politico explains.

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CanSino and Oxford-AstraZeneca already have large-scale Phase III trials underway to judge their vaccines' efficacy, and CanSino's is the only COVID-19 vaccine approved for use, though approval is limited to China's military. Monday's reports show that "each of these vaccines is worth taking all the way through to a Phase III study," Dr. Peter Jay Hotez, a vaccine researcher at the Baylor College of Medicine, tells The New York Times. "That is it. All it means is 'worth pursuing.'"