In their study, leading epidemiologists Benjamin Cowling and Gabriel Leung, and virologist Malik Peiris wrote: “The difference in concentrations of neutralising antibodies identified in our study could translate into substantial differences in vaccine effectiveness.”

In an exclusive interview with the Post, lead researcher Cowling noted that moderate levels of protection were still better than none at all, and urged Hongkongers to get vaccinated.

“Don’t let the perfect be the enemy of the good,” he said.

For the antibody study, HKU researchers recruited 1,442 health care workers from public and private hospitals and clinics around the city, and collected blood samples from them at various intervals.

In the first batch of 93 fully vaccinated participants, antibody concentrations for the 63 BioNTech recipients rose “substantially” after the first dose and again after the second. The 30 Sinovac recipients, meanwhile, had “low” antibody concentrations after their first jab, and “moderate” ones after the second.

Using a “gold standard” method known as PRNT, Cowling said, researchers found the average antibody levels of a subset of 12 BioNTech recipients to be 10 times higher – at 269 – than those of an equal number of people in the Sinovac group, which clocked in at 27.

The researchers also found that the BioNTech recipient with the lowest level of antibodies still had more than the Sinovac taker with the highest.

Sinovac-generated antibody levels were judged to be about the same as, or slightly lower than, those detected in recovered Covid-19 patients.

Cowling said the gap between the two drugs was consistent with the discrepancies reported by the manufacturers in their third-phase clinical trials, which found BioNTech to 95 per cent effective, compared to 50.7 per cent for Sinovac.

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Still, the latest study provided a rare head-to-head comparison of the two vaccines, he added. That made it valuable to scientists given that the manufacturers’ trials involved different sample groups and sizes, making direct comparisons of their results difficult.

The findings suggested a potential need for Sinovac takers – especially the elderly, who had weaker immune responses to vaccines – to get a third shot as a booster, Cowling said.

Cowling said the recipients of both vaccines had detectable antibodies one month after their second jabs, but he could not say for certain what that might mean for a government plan to shorten quarantine periods for some vaccinated travellers if they could show proof of antibodies. It was not yet clear, he noted, whether some commercial labs would be sensitive enough to pick up low antibody levels.

A previous study on antibodies by the Hong Kong Sanatorium and Hospital, published in the Hong Kong Medical Journal last month, delivered similar findings.

That study, which recruited 457 vaccinated health care workers, used different approaches to measure antibody levels, with one also showing that BioNTech recipients had 10 times more of the proteins than those who got Sinovac.

But the researchers noted that more research was needed, as the level of antibodies required to confer protection against future infection was currently “not well defined”.

Additional reporting by Elizabeth Cheung