A new Brazilian trial of one of China’s COVID-19 vaccines found that the treatment was only effective about 50 percent of the time. The CoronaVac vaccine was developed by China’s Sinovac Biotech company. On Tuesday, researchers said a new late-stage trial in Brazil showed the vaccine was just 50.4 percent effective at preventing symptomatic infections. The Brazilian trial included data on what the researchers called “very mild” COVID-19 cases. Last week, researchers said the vaccine had performed with 78 percent effectiveness against “mild to severe” cases. CoronaVac is one of several vaccines being developed by Chinese companies.