China has approved five Chinese-made vaccines. Three use an inactivated pathogen to trigger an immune response. Two of them are made by Sinopharm and one by Sinovac. Professor Wu Zhiwei, a public health expert from Nanjing University's Medical School, shares his insights on mixing vaccines to boost efficacy. He said, theoretically, it is possible to mix two vaccines to gain better immunity, but only science can address that. He said safety is the primary concern.