COVID-19 cases are still surging across the U.S. As a result, several local authorities in the U.S. have re-introduced mask mandates for indoor settings. Professor Wu Zhiwei in Nanjing, an expert from the Medical School of Nanjing University, shares his view on the COVID-19 situation in the U.S. He says that evidence shows vaccines still protect against Delta variant, but with reduced efficacy, an extra shot may help.