A third shot of Sinopharm's Covid-19 vaccine leads to a rebound in antibody levels that drop months after a second dose, a small-scale Chinese study showed.

It also showed the booster enhanced cell-based responses to the coronavirus.

About five months after a second Sinopharm dose, the average concentration of neutralising antibody against the virus dropped by 70 per cent from the level seen four weeks after that shot, according to analysis of samples taken from vaccinated healthcare workers.

But one week after a third shot, the antibody concentration had increased 7.2-fold compared with the level seen five months after the second shot, researchers from a hospital affiliated with the Sun Yat-sen University said in the paper.