SERRANA, Brazil—This city of 45,000 people in southern Brazil has been at the center of a unique experiment the past three months: Vaccinate nearly every adult against Covid-19 and see what happens.

In recent weeks, after the vast majority of adults here got their second dose, cases and deaths plunged and life here has started to return to normal, even as the pandemic continues to rage across Brazil.

“We feel free,” said Homero Cavalheri, a 68-year-old retired architect.

The experiment gives hope to countries still battling outbreaks that mass inoculation works. It also offers new evidence of the efficacy of Sinovac’s shot being used by dozens of developing nations.

All of Serrana’s adults were offered CoronaVac between February and April as part of the experiment, baptized Project S, the first mass trial of its kind against Covid-19 in which an entire town is vaccinated before the rest of the country.

Minors under 18, pregnant or nursing adult women and some others with serious health issues weren’t eligible. But of the roughly 27,700 eligible adults, 27,150 got vaccinated, say town officials—98% of eligible adults.

The Butantan Institute, the public-research center that is producing CoronaVac in Brazil and running the experiment, declined to comment until the full results of the mass trial are released later this month.

But city officials, and residents, say they are thrilled with the results so far. Infections are down 75% from a March peak, while there have been no deaths from Covid-19 among the people here who were fully vaccinated, suggesting CoronaVac also is effective against the aggressive P.1 variant sweeping the region.

“The numbers speak for themselves,” Serrana’s mayor, Léo Capitelli, said. “It worked!”

In the waiting room of the town’s intensive-care unit, the effects of CoronaVac are visible. “Only three weeks ago, this was so full people had to stand up,” said Lucia Elaine Caldano, the unit’s administrator, pointing to rows of empty chairs. In the past three weeks, only one person has been put on a ventilator—a woman who refused to get the shot.

It’s a different story across Brazil, where only 7% have been vaccinated, and almost a hundred people are dying an hour from the disease, while thousands more languish in overcrowded hospitals.

After registering an average of 67 new infections a day in March, Serrana’s average daily case tally this month has been a quarter of that, about 17 a day. Meanwhile, Brazil’s infection rate has fallen only about 20% since its March peak.

One of the biggest surprises, Serrana’s officials said, was that nearly everyone agreed to take the shot. In a nationwide survey in December conducted by a leading Brazilian pollster, half of respondents said they would refuse to take a Chinese vaccine.

Serrana’s experiment promises to add clarity to a mishmash of studies world-wide into CoronaVac, authorized for use in more than 30 countries.

Serrana’s experiment also provides some of the first real-world evidence that CoronaVac is effective against the aggressive P.1 strain that was responsible for at least 60% of the town’s infections in March.

Serrana was chosen because it was a small commuter town with a high infection rate—about a quarter of residents travel to Ribeirão Preto for work each day, allowing the virus to circulate easily.

China’s Sinovac provided the doses at no cost. With the pandemic largely under control in the Asian nation, Beijing has looked to hard-hit countries to test its vaccines.

When Butantan first announced Project S, it said it hoped the results would reassure Brazilians that the vaccine is both safe and it works.

Rumors have raged about the safety of CoronaVac after President Jair Bolsonaro, a fierce China critic, told supporters last year it could disable or even kill them, without offering evidence.