Clinical trials in Brazil of a Chinese-made vaccine against Covid-19 have shown "extremely positive" results, and a widespread vaccination campaign could begin as early as December, the governor of Sao Paulo state said Wednesday. Sao Paulo, the epicenter of the coronavirus pandemic in hard-hit Brazil, is one of six states helping to test the so-called CoronaVac vaccine developed by Chinese pharmaceutical firm Sinovac Biotech. The vaccine produced an immune response in 98 percent of recipients over 60 years old, with no adverse side-effects reported so far, said Governor Joao Doria. "The results have been extremely positive," he told a news conference. "We will soon be able to immunize Brazilians in Sao Paulo and across the country with the CoronaVac vaccine.... The projected delivery date is in December this year." Sinovac has partnered with a Brazilian public health research center, the Butantan Institute, to conduct Phase 3 clinical trials of the vaccine -- the last step before regulatory approval. The deal gives the institute the right to produce 120 million doses of the vaccine, according to officials. CoronaVac has gotten caught up in a political battle in Brazil, however. President Jair Bolsonaro, whose administration has tense relations with China, has criticized the vaccine, and lashed out at Doria, a leading opponent, for supposedly backing it. The far-right president has instead allocated 1.9 billion reals ($360 million) to purchase another vaccine candidate, developed by Oxford University and pharmaceutical firm AstraZeneca. Trials of that vaccine, which is also being tested partly in Brazil, were suspended Tuesday after a volunteer recipient developed an unexplained illness -- a move the company described as "routine." Brazil has the second-highest death toll in the pandemic after the United States, with more than 127,000 people killed and 4.1 million infections. The South American country has emerged as a leading testing ground for vaccines. In the latest development, Brazilian medical diagnostics company Dasa and US vaccine-maker COVAXX announced a deal Wednesday to conduct Phase 2 and 3 trials of the latter's Covid-19 vaccine in Brazil. COVAXX, a subsidiary of US firm United Biomedical, plans to test the vaccine on at least 3,000 volunteers in Brazil.