People head to a mass COVID-19 vaccination site at the United Center in Chicago, the United States, on March 10, 2021. The mass COVID-19 vaccination site started running at the arena on Tuesday. (Photo by Joel Lerner/Xinhua)

People leave a mass COVID-19 vaccination site after getting vaccinated at the United Center in Chicago, the United States, on March 10, 2021. The mass COVID-19 vaccination site started running at the arena on Tuesday. (Photo by Joel Lerner/Xinhua)

A man heads to a mass COVID-19 vaccination site at the United Center in Chicago, the United States, on March 10, 2021. The mass COVID-19 vaccination site started running at the arena on Tuesday. (Photo by Joel Lerner/Xinhua)

A security guard directs people to the check-in at a mass COVID-19 vaccination site at the United Center in Chicago, the United States, on March 10, 2021. The mass COVID-19 vaccination site started running at the arena on Tuesday. (Photo by Joel Lerner/Xinhua)

People head to a mass COVID-19 vaccination site at the United Center in Chicago, the United States, on March 10, 2021. The mass COVID-19 vaccination site started running at the arena on Tuesday. (Photo by Joel Lerner/Xinhua)

A man has his temperature taken before entering a mass COVID-19 vaccination site at the United Center in Chicago, the United States, on March 10, 2021. The mass COVID-19 vaccination site started running at the arena on Tuesday. (Photo by Joel Lerner/Xinhua)

People line up to enter a mass COVID-19 vaccination site at the United Center in Chicago, the United States, on March 10, 2021. The mass COVID-19 vaccination site started running at the arena on Tuesday. (Photo by Joel Lerner/Xinhua)