Antibody levels in people inoculated with CanSino Biologics' (CanSinoBIO) single-dose COVID-19 vaccine fell by some 30% after six months, however a booster shot could offer a significant lift, a senior executive said late on Thursday.

The decline in antibodies does not necessarily mean the shots will lose their protection, though how fast they wane could still serve as an important indicator of the immune response, Zhu Tao, chief scientific officer at China's CanSinoBIO, said in an online presentation.

Apart from durable antibodies, researchers have said that other components in a person's immune system, such as T cells and B cell memory, elicited by COVID vaccines may also contribute to protection.

Six months after being vaccinated with CanSinoBIO's shot, neutralising antibodies were at roughly 70% of the level seen 28 days after the shot, Zhu said, citing data from 35 vaccinated people.

When a second dose was given after six months, antibody levels shot up about eight fold two weeks after the shot, Zhu said, citing data from 28 people.

Five out of the seven vaccines approved in China are two-shot inactivated vaccines from Sinovac and state-backed Sinopharm that contain "killed" coronavirus. CanSinoBIO's shot uses a modified common cold virus known as adenovirus to ferry genetic information from the spike protein of the coronavirus into the human body.

A third booster shot of CanSinoBIO's vaccine three to six months after the second shot of an inactivated vaccine generated significantly higher antibody levels, versus using an inactivated vaccine as a third-dose booster, Zhu said. He cited data from about 200 participants in a clinical trial.