In a serious setback to its effort to emerge as the dominant vaccine supplier to developing countries, Brazilian researchers on Tuesday (Jan 12) announced that China’s Sinovac vaccine was far less effective than originally touted, The Wall Street Journal reported.

According to data made available by Brazil's Butantan Institute, a São Paulo-based public institute, the efficacy of Sinovac’s CoronaVac in a late- stage trials was at just 50.38 percent, nearly 30 percentage points lower than what the initial data showed.

The vaccine narrowly meets the 50 percent threshold required for approval by Brazilian regulators and the World Health Organization (WHO).

The institute had earlier announced last week that late-stage trials had found the CoronaVac vaccine to be between 78 and 100 percent effective.

After coming under severe criticism from number of scientists who accused the trial’s organizers of misleading the public, the institute revealed researchers had separated the cases into six categories: asymptomatic, very mild, mild, two levels of moderate, and severe. The earlier rates included only volunteers who had mild to severe cases of coronavirus.

Given that CoronaVac was100 percent effective in preventing severe cases, experts are of view that it is still more effective than some flu vaccines. It can be stored inexpensively in a regular refrigerator which will make it useful option for distribution in developing countries.

The controversy over the efficacy of CoronaVac is likely to pose a challenge to Brazil given the high level of vaccine skepticism prevailing in the country.

Brazil’s health regulator are still reviewing a request for emergency use of CoronaVac.