However, they added that they can't determine whether or not this positive discovery is down to him receiving a shot of Sinovac on top of his two BioNTech jabs, RTHK reports.

The 32-year-old man first had a dose of BioNTech in March. But his second dose was delayed when vaccinations with the German-made jab were temporarily halted due to packaging defects.

When he couldn't get the BioNTech jab as planned, the man decided to go for a Sinovac one instead.

Then, when use of BioNTech resumed in Hong Kong, he took a second dose of this one as well.

Professor Ivan Hung from the University of Hong Kong said antibody levels vary among vaccinated people as their immune systems react differently.

He said although tests showed the man has an antibody level four times higher than average, some people who only receive two shots of BioNTech can also reach that level.

“That could be the result of two doses of BioNTech or it could be the result of three doses of Covid-19 vaccine. Unfortunately, we did not have the antibody level after the second dose, so we cannot draw any conclusion just by looking at the antibody level,” he said.

“If you compare to the highest level of the subjects who received two doses of BioNTech, it is comparable.”

Hung said people who have received two doses of coronavius vaccine, produced by either BioNTech or Sinovac, will have adequate antibodies and they should last for nine months to a year.

He said people do not need a third dose of vaccine, but will probably require a booster jab next year.