ISTANBUL -- Researchers in Turkey on Wednesday reported an efficacy rate of 83.5% for the coronavirus vaccine developed by Chinese company Sinovac Biotech based on final Phase 3 clinical trial results there, down from 91.25% announced in late December from interim results.
Professor Serhat Unal, a member of Turkey's COVID-19 advisory board and a leader of the clinical trial, told Nikkei Asia that data was used from 10,030 volunteers in the study out of 10,216 participants. The interim results covered only 1,322 volunteers. The final rate of 83.5% is based on 41 infections, in which 32 of the individuals had received a placebo.
The efficacy was derived from the rate of volunteers who returned a positive result in a polymerase chain reaction (PCR) test and also developed at least one COVID-19 symptom such as fever, cough or headache at least 14 days after receiving the second dose.
Sinovac's CoronaVac vaccine was 100% effective in preventing severe illness that led to hospitalization, researchers said. All six hospitalized participants were in the placebo group. The trial age group ranged from 18 to 59, said researchers, who noted they have concluded no study of CoronaVac regarding mutations of the coronavirus.
"I can tell the international community confidently that the vaccine is effective and safe," Unal told Nikkei.
Yet Sinovac announced on Feb. 5 that Phase 3 clinical trials by Brazil's Butantan Institute on over 12,000 health care workers showed a general efficacy rate of 50.65%. The results in Brazil also showed efficacy of 83.7% for cases requiring medical treatment and 100% for hospitalization as well as severe and fatal cases.
The discrepancy in results coming from studies in different countries prompted criticism.
"Unlike global pharmaceutical companies' centralized clinical trial protocols, Chinese vaccine producers left the clinical trial protocol setting to each applier country's discretion," said professor Ali Yagiz Uresin, head of pharmacology and the clinical pharmacology department at Istanbul University's Istanbul Faculty of Medicine. This approach produced "very different scopes, volunteer profiles and case definitions, which caused very different clinical trial results in different countries, confusing public opinion."
"Unlike Turkey, Brazil's CoronaVac Phase 3 clinical trial was carried out entirely on health care workers, who are much exposed to the coronavirus," Uresin said. "Moreover, in Brazil's case there is a general efficacy segment which was slightly above the 50% mark, showing all PCR positive results, while Turkey's study is based on PCR positive cases as well as showing at least one symptom. Indeed, both Turkish and Brazilian studies show the vaccine is efficient and results are actually similar."
Turkey so far has received 15 million doses of the vaccine from Sinovac, out of a contract for 100 million doses through the end of May.
Turkish authorities gave emergency use approval for CoronaVac in January, following the interim results in late December. Turkey has given over 9.4 million doses to nearly 7.3 million of the country's 83 million residents.