The coronavirus vaccine developed by China's Sinovac has been found to be 50.4% effective in Brazil, according to the latest results.
Researchers said during the clinical trials they found the vaccine is significantly less effective than previous data suggested - barely over the 50% needed for regulatory approval.
The Chinese vaccine is one of two that the Brazilian government has lined up.
Brazil has been one of the countries worst affected by Covid-19.
Sinovac, a Beijing-based biopharmaceutical company, is behind CoronaVac, an inactivated vaccine.
It works by using killed viral particles to expose the body's immune system to the virus without risking a serious disease response.
Several countries, including Indonesia, Turkey and Singapore, have placed orders for the vaccine.