Staff members work at a drive-thru vaccination site in San Francisco, the United States, May 22, 2021. (Xinhua/Wu Xiaoling)

A man waits to get administered against COVID-19 in his vehicle at a drive-thru vaccination site in San Francisco, the United States, May 22, 2021. (Xinhua/Wu Xiaoling)

Photo taken on May 22, 2021 shows a drive-thru vaccination site in San Francisco, the United States. (Xinhua/Wu Xiaoling)

Photo taken on May 22, 2021 shows a drive-thru vaccination site in San Francisco, the United States. (Xinhua/Wu Xiaoling)

Photo taken on May 22, 2021 shows a drive-thru vaccination site in San Francisco, the United States. (Xinhua/Wu Xiaoling)