A booster dose of Sinovac Biotech's Covid-19 vaccine reversed a decline in antibody activities against the Delta variant, a study showed, easing some concerns about its longer-term immune response to the highly contagious strain of the virus.

Neutralising antibody activities against Delta were not detected in samples taken from vaccine recipients six months after they received the second dose of Sinovac's CoronaVac vaccine, according to the study.

But recipients of booster shots showed over 2.5-fold higher neutralising potency against Delta about four weeks after the third dose, compared with the level seen about four weeks after the second shot, researchers from Chinese Academy of Sciences, Fudan University, Sinovac, and other Chinese institutions said in the paper.