Brazil, where the number of cases has now surpassed 2 million, is one of a small handful of sites for testing experimental coronavirus vaccines. It offers an unusual and appealing mix for research: a skyrocketing rate of transmission, as well as internationally respected research centers and a public health system experienced in creating and distributing vaccines.

According to the World Health Organization, as of July 14, 163 Covid-19 vaccines were being developed around the world, and 23 of them had started clinical trials involving humans. But only two have reached Phase 3 -- the last scientific stage before approval to be marketed -- which requires large-scale trials with thousands of individuals to assess the efficacy and safety of the vaccine.

Both Phase 3 trials will include Brazil and are scheduled to involve at least 14,000 Brazilians. Advanced talks are also underway to launch three more vaccine trials in the country, according to Brazilian institutes consulted by CNN.

Why test in Brazil?

While Brazil's president, Jair Bolsonaro, has repeatedly downplayed the virus as a "little flu" and been criticized by experts for his unwillingness to implement forceful nationwide containment measures, the vaccine research being carried out now inside Brazil's borders could prove to be a global game changer as the northern hemisphere braces for a possible second wave in the winter.

Julio Barbosa, a 42-year-old nursing technician who has already lost five colleagues to the coronavirus, has volunteered to participate in one of the mass vaccination trials, conducted by Oxford University and pharmaceutical company AstraZeneca. The Phase 3 trial will involve 50,000 volunteers worldwide.

After taking the shot, Barbosa says he had a low-grade fever and mild muscular pain which disappeared the next morning. In the trial, involving mostly health workers, half of the volunteers are receiving the trial Covid-19 vaccine and half are receiving a meningitis vaccine, which can provoke similar symptoms.

"This vaccine has to come out soon so we can have a break at the hospital. I haven't stopped working in the last four months," he told CNN after receiving a shot at a medical warehouse erected in Sao Paulo for the trial.

Chinese biotech company Sinovac is also beginning a Phase 3 trial in Brazil, in collaboration with Brazil's Butantan Institute in Sao Paulo. Its CoronaVac trial vaccine uses inactivated virus cells to stimulate an immune response in patients. The tests will begin next Monday with 9,000 volunteers in five Brazilian states plus the capital.

Like the Oxford vaccine, CoronaVac will be given mostly to health professionals. Ricardo Palacios, medical director of research at Butantan, says that the institute is also in "very advanced conversations with two other vaccines under development" and in talks with dozens of pharmaceutical companies about Covid-19 research studies.

"All producers in the world will always look for a place where there is high transmission to

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