The leaders of a Chilean late-stage human trial of the CoronaVac Covid-19 vaccine developed by China's Sinovac on Thursday recommended a third dose of the jab to protect against the more contagious Delta variant.

The trial leaders said a separate in vitro laboratory trial to determine the vaccine's effectiveness against the Delta strain of the virus showed that neutralising antibodies reduced four-fold compared to those produced against the original coronavirus strain first found in China...