China's drug regulator has approved the country's first mixed-vaccine trial, a company involved in the study said, as the rapid spread of the Delta variant raises concern about the efficacy of domestically produced jabs, Reuters reports.

The trial will test the efficacy of combining an “inactivated” vaccine made by China's Sinovac with a DNA-based one developed by US pharmaceutical company Inovio, a statement issued on Tuesday said. The statement was put out by Advaccine Biopharmaceuticals Suzhou, Inovio's trial partner in China.

Preclinical work has found that “two different vaccine applications...produce an even stronger and more balanced immune response”, Advaccine chairman Wang Bin said in the statement.

The World Health Organisation has said there is still not enough data to say whether using two different vaccines together is safe or can boost immunity.