Sofia MoutinhoScience’s COVID-19 reporting is supported by the Heising-Simons Foundation.A small commuter town surrounded by sugarcane fields in southeastern Brazil, one of the countries hardest hit by COVID-19, has shown that even a vaccine that had low efficacy in some clinical trials can dramatically control the pandemic virus.As part of an unusual experiment to track the real-world effectiveness of CoronaVac, a COVID-19 vaccine made by a Chinese company, almost all adult residents of Serrana, in the state of São Paulo, received the required two shots between February and April, long before most would otherwise have become eligible for the vaccine.