Clinical trials of the CoronaVac coronavirus vaccine developed by Chinese laboratory Sinovac have "reached the efficacy threshold" demanded by the World Health Organization, the Brazilian institute charged with its production and distribution said on Wednesday.

However, the Butantan Institute didn't publish the results of those trials -- the last before authorization.

"We've reached the efficacy threshold that allows us to seek an emergency use" authorization from Brazil's Anvisa regulatory agency, said Butantan director Dimas Covas.

He said a clause in the Sinovac contract insisted that these phase III test results could only be made public once all clinical trials around the world have been completed.

In Brazil, 13,000 volunteers took part in the trials, which were also performed in Turkey, Indonesia and China.

"Our entire program remains unchanged. The production of doses is continuing in our factory and the immunization campaign will begin as expected on January 25," said Sao Paulo state health secretary Jean Gorinchteyn.

The WHO's efficacy threshold is 50 percent but other laboratories, such as Pfizer and BioNTech that are collaborating on a vaccine that has already been administered to thousands of people in the United States and Britain, have reached 90 percent efficacy.

Covas said the federal government would order 100 million CoronaVac doses, more than double the 46 million initially publicized.

Immunization has been a highly politicized issue in Brazil where far-right President Jair Bolsonaro has repeatedly said he won't take a vaccine while he's also tried to discredit the CoronaVac jab.

The Butantan Institute is supported by Sao Paulo state, whose governor Joao Doria repeatedly clashed with Bolsonaro over the country's coronavirus response and is expected to challenge the incumbent in the 2022 presidential elections.

Brazil has suffered the second-largest number of coronavirus deaths in the world after the US with 188,000 dead.