A COVID-19 vaccine developed by China’s Sinovac Biotech was reported to be 91.25% effective during a late-stage trial in Turkey, but a Hong Kong expert cast doubts on the efficacy, citing that the sample size was too small.

Turkish health authorities on Thursday announced the interim data for Phase III research of the Sinovac vaccine based on only 1,322 of the more than 7,000 study participants.

Researchers at the Butantan Institute in Brazil, which is also running a final Phase III trial of the vaccine, said on Thursday that Sinovac requested a 15-day delay in releasing data of the vaccine’s effectiveness to allow time to consolidate global test results. The shot was more than 50% effective, researchers had said on Wednesday.

Turkish researchers said 26 of the 29 people who were infected during the trial were given placebos. One showed allergic reactions but no serious side effects, while others had side effects including fever, slight pain and fatigue. They added that the trial would continue until 40 people become infected.

The Phase III data for the Sinovac vaccine was still preliminary and could not be fully trusted, since it only involved one test center and a small number of participants, respiratory medicine specialist Leung Chi-chiu said.

Leung said he was “cautiously optimistic” about the vaccine as he was still unsure about how long it could last for and whether it would cause serious side effects.

Leung said he would not speculate as to why Sinovac delayed the release of Phase III data twice. Brazil and Turkey have shown confidence in the vaccine, and it would raise an alarm if Brazil didn’t buy it, he added.

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