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

In recent weeks, after most of the adults here got their second dose, Covid-19 cases and deaths plunged and life has started to return to normal as the pandemic continues to rage across Brazil.

In the heart of Serrana, children squealed with laughter as they chased each other across the main square, while groups of friends—many unmasked —stopped to chat and bask in the afternoon sun.