(Bloomberg) -- Brazil officials said a shot developed in partnership with Sinovac Biotech Ltd was 50.38% effective in preventing coronavirus cases, a number that meets the threshold required by global regulators for approval but that’s well below the 78% figure announced last week.

At a press conference Tuesday, Sao Paulo state officials and researchers from Butantan Institute explained that they split the cases in six categories: asymptomatic, very mild, mild, two levels of moderate, and severe -- the first two not requiring medical assistance.

The 78% was calculated considering the mild, moderate and severe cases, officials said. When very mild cases are included among the 13,000 volunteers, the figure is 50.4% -- 167 infected volunteers in the placebo arm, and 85 in the vaccine arm. The shot proved 100% effective in preventing severe cases.

Brazilian regulators and the World Health Organization require vaccines to be 50% effective for approval.

Localized pain was the biggest side effect of the shot, as well as headaches and fatigue. There haven’t been any serious adverse effects recorded, according to Butantan, which is developing the shot locally. Seven volunteers in the placebo arm of the trial needed hospitalization, while none in the group who received the shot did.

Data Kerfuffle

Last week, officials had said the so-called CoronaVac vaccine was 78% effective in preventing mild cases of Covid-19 and 100% effective against severe and moderate infections. At the time, it wasn’t apparent how the researchers had calculated the efficacy rate, as officials didn’t provide a more detailed breakdown of the study. Based on the information available, observers had placed the efficacy rate around 62.5%.

China’s Covid Vaccine Has Four (Wildly) Different Efficacy Rates

The confusion over the data is fueling skepticism over Chinese vaccines. Turkey and Indonesia, which are also testing the shot, have reported different efficacy numbers and plan to begin administering the immunization this week.

In Brazil, a poll showed half of the population doesn’t want to get the shot -- the highest amount out of the vaccines available.

Brazil’s health regulator is still reviewing a request for emergency use of CoronaVac. Sao Paulo authorities say they plan to begin vaccinations on Jan. 25.

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