BEIJING • 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.

The study comes amid concerns about the Chinese vaccine's efficacy against Delta, which has become the dominant variant globally and is driving a surge in new infections even in the most vaccinated countries.

Several countries which relied heavily on the Sinovac vaccine have begun giving booster shots developed by Western manufacturers to people fully vaccinated with the Chinese shot.

Neutralising antibody activity against Delta was not detected in samples from vaccine recipients six months after they got the second dose of Sinovac's vaccine, according to the study published on Sunday before a peer review.

But recipients of booster shots showed an 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 the Chinese Academy of Sciences, Furan University, Sinovac and other Chinese institutions said in the paper.

They did not discuss how specifically changes in antibody activity will affect Sinovac's efficacy in preventing people getting sick from the variant.

The lab study involved samples from 66 participants, including 38 volunteers who received either two or three doses of the vaccine.

The World Health Organisation plans to ship around 100 million doses of the Sinovac and Sinopharm shots by the end of this month, mostly to Africa and Asia, in its first delivery of Chinese vaccines.

REUTERS