Officials at the Butantan Institute in São Paulo said on Tuesday that a trial conducted in Brazil showed that the CoronaVac vaccine, manufactured by the Beijing-based company Sinovac, had an efficacy rate just over 50 percent. That rate, slightly above the benchmark that the World Health Organization has said would make a vaccine effective for general use, was far below the 78 percent level announced last week.

The implications could be significant for a vaccine that is crucial to China’s global health diplomacy. At least 10 countries have ordered more than 380 million doses of the Sinovac inoculation, CoronaVac, though regulatory agencies have yet to fully approve it.

A senior official in Hong Kong, a special administrative region of China that had already ordered CoronaVac, said on Wednesday that an advisory panel would strictly review the vaccine based on clinical trial data before it was rolled out there.