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

Neutralizing 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 published on Sunday before a peer review.

But recipients of booster shots showed over 2.5-fold higher neutralizing 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, Furan University, Sinovac, and other Chinese institutions said in the paper.

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

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

The World Health Organization plans to ship around 100 million doses of the Sinovac and Sinopharm COVID-19 shots by the end of this month, mostly to Africa and Asia, in its first delivery of Chinese vaccines.

However, some countries rejected accepting the vaccines, citing lack of data on their effectiveness against Delta.

South Africa is listed by COVAX as one of Africa’s largest recipients of Chinese shots with an allocation of 2.5 million Sinovac doses, but a senior health official said the country was currently unable to accept the vaccines.

COVAX’s allocation of Chinese vaccines comes after the WHO gave emergency approval to the Sinopharm shot in May and Sinovac in June.

Around 1.8 billion doses of the Sinovac vaccine have been supplied globally including China by end August, according to the company.

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