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...