RIO DE JANEIRO — The chaotic response to the coronavirus in Brazil, where it has killed more than 105,000 people, made the country’s experience a cautionary tale that many around the world have watched with alarm.

But as the country’s caseload soared, vaccine researchers saw a unique opportunity.

With sustained widespread contagion, a deep bench of immunization experts, a robust medical manufacturing infrastructure and thousands of vaccine trial volunteers, Brazil has emerged as a potentially vital player in the global scramble to end the pandemic.

Three of the most promising and advanced vaccine studies in the world are relying on scientists and volunteers in Brazil, according to the World Health Organization’s report on the progress of vaccine research.

The embattled government hopes its citizens could be among the first in the world to be inoculated. And medical experts are imagining the possibility that Brazil could even manufacture the vaccine and export it to neighboring countries, a prospect that fills them with something that has been in short supply this year: pride.

“I’m very optimistic,” said Dimas Covas, the director of the Butantan Institute, an internationally renowned biopharmaceutical producer that is partnering with China’s Sinovac on one of the studies that has reached the third stage of research, during which potential vaccines are tested on 9,000 people.

“Brazil will be one of the first countries to have the vaccine.”

Some 5,000 Brazilians have also been recruited to support a vaccine trial conducted by AstraZeneca, a British-Swedish pharmaceutical company in partnership with Oxford University. An additional 1,000 volunteers in Brazil were recruited to test a vaccine developed by New York-based Pfizer.

Researchers need countries with large enough outbreaks to assess whether a vaccine will work. Some volunteers are given the potential vaccine while others are given a placebo, but they have to be in a place where enough virus is circulating to test the vaccine’s efficacy.

Brazil, where the virus has infected more than three million people, has clear conditions for these trials. And it will be the only country other than the United States to be playing a major role in three of the leading studies as an unparalleled quest for a vaccine has led to unusually fast regulatory approvals and hastily brokered partnerships.

Still, it is far from certain, experts say, that the vaccine trials underway in Brazil will win the race.

Countries across the world are vying to be among the first to get access to a vaccine that will be in demand by billions of people. In India, one of the country’s wealthiest families is taking a gamble by mass producing the Oxford vaccine in hopes that it will be the first to clear safety and regulatory hurdles.

Russia this week approved a homemade vaccine which has not yet met the final tests for safety and efficacy. If it works, it could position the country to claim it developed the world’s first effective coronavirus vaccine.

Brazil’s explosive caseload has made it the second hardest-hit nation in the world after the United States. While other countries in the region have higher per capita rates, experts have assailed President Jair Bolsonaro’s cavalier handling of the crisis.

The president, who caught the virus in July, has called it a “measly flu” and sabotaged calls for quarantines and lockdowns. He also appointed an Army general with no medical experience to run the health ministry after two ministers clashed with the president over his disdain for science-based approaches.

Because of the country’s disorganized response to the virus, Brazilians have been subjected to travel bans, neighbors have militarized border crossings and unions representing medical workers recently asked the International Criminal Court to charge Mr. Bolsonaro for crimes against humanity, arguing that he has given the virus free rein.

Brazil has a universal public health care system with one of the best immunization programs in the developing world, which has enabled it to contain outbreaks of yellow fever, measles and other pathogens.

But in recent years, as the economy has contracted, the program has suffered, dogged by budget cuts. It has also had to fight disinformation campaigns that have found a rapt audience on social media.

In 2019, for the first time in 25 years, Brazil didn’t fulfill its vaccination goal for any of the shots it routinely administers.

A coronavirus breakthrough could galvanize the country’s vaccine sector. It could also invigorate its scientific institutions, which employ world class scientists but have been reeling after years of budget cuts that have weakened the public health care system and dented the country’s reputation as a research powerhouse.

Katherine O’Brien, the director of immunization at the World Health Organization, welcomed Brazil’s investments in manufacturing vaccines for Covid-19, the disease caused by the virus. But she said bilateral deals like the ones Brazil is involved in were still a gamble.

“Some countries are going to be lucky, entering into contracts with a candidate that’s going to demonstrate efficacy,” Dr. O’Brien said. “Other countries are going to pursue deals with candidates that are going to fail and they’ll get nothing.”

Home to about 210 million people, Brazil has the capacity to make roughly 500 million vaccines per year. Under the current coronavirus vaccine deals Brazil has stakes in, Brazilian vaccine plants would initially handle the final stages of vaccine production after importing the raw materials, and later produce them entirely.

Brazil has signed two deals to get preferential access to a vaccine. One, between the São Paulo state’s Butantan Institute and Sinovac, would provide Brazilians with 120 million doses of the vaccine by early 2021. The second one, between the federal government’s Bio-Manguinhos and AstraZeneca, guarantees access to 100 million doses of the vaccine by the beginning of next year.

Both deals include a technology transfer agreement that would allow Brazil to later manufacture vaccines on its own. Government officials hope to start vaccinating some Brazilians by the first semester of 2021, though an exact date depends on the results of ongoing studies and a future approval process with the local regulatory agency.

Carla Domingues, an epidemiologist who ran the country’s immunization program until last year, said disinformation campaigns about immunization have hobbled efforts to protect people from HPV, a sexually transmitted infection.

“Unfortunately, this trend that we have been seeing in other countries for many years is now here in Brazil,” she said. “And we haven’t managed to reverse it.”

Yet recruiting volunteers for the ongoing studies in Brazil has not been a challenge, said Soraya Smaili, the president at the Federal University of São Paulo, which is involved in the AstraZeneca and Oxford study.

“It has not been hard to find volunteers,” she said. “People have stepped forward and everyone wants to be part of the solution. This has been a lovely social movement.”

Denise Abranches, a dental surgeon who has spent months treating coronavirus patients with mouth sores in intensive care units, was among the first to volunteer for a vaccine. She said her only fear was not getting in line soon enough to get a shot.

“I see this as a way for us to regain a leadership role” in the global scientific community, she said. “The world is looking at us for answers and this is a vaccine that could help everyone in the world.”

Maurício Zuma, the director at Bio-Manguinhos, one of the manufacturers that hopes to produce Covid-19 vaccines in Brazil, said that after the country meets its internal demand, he hopes to export vials to neighboring countries that also have been struggling with large caseloads.

“Our intention is to take part in a movement of solidarity,” he said. “If we manage to produce the vaccine here and end up with a surplus, we’re obviously going to make sure it’s used in other countries in Latin America.”

But as researchers celebrate Brazil’s role in the global vaccine race, they have also felt compelled to remind citizens that the good news won’t single-handedly put an end to the suffering the virus has unleashed in the country.

“They should not assume that that’s it and they are done,” said Maria Elena Bottazzi, a vaccine developer at Baylor College of Medicine. “There is still a lot of work that Brazil needs to do to strengthen their public health infrastructure to reduce the transmission of the virus.”