People wearing face masks are seen near the Lincoln Memorial in Washington, D.C., the United States, on Jan. 1, 2021. The total number of COVID-19 cases in the United States topped 20 million on Friday, according to the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University.

People wearing face masks visit the National Mall in Washington, D.C., the United States, on Jan. 1, 2021. The total number of COVID-19 cases in the United States topped 20 million on Friday, according to the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University.

People wearing face masks visit the Lincoln Memorial in Washington, D.C., the United States, on Jan. 1, 2021. The total number of COVID-19 cases in the United States topped 20 million on Friday, according to the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University.

People wearing face masks visit the Lincoln Memorial in Washington, D.C., the United States, on Jan. 1, 2021. The total number of COVID-19 cases in the United States topped 20 million on Friday, according to the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University.

People visit the National Mall in Washington, D.C., the United States, on Jan. 1, 2021. The total number of COVID-19 cases in the United States topped 20 million on Friday, according to the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University.

Healthcare workers wheel a patient into a hospital in New York, the United States, Jan. 1, 2021. The total number of COVID-19 cases in the United States topped 20 million on Friday, according to the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University.

Healthcare workers wheel a patient into a hospital in New York, the United States, Jan. 1, 2021. The total number of COVID-19 cases in the United States topped 20 million on Friday, according to the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University.

Pedestrian wearing face coverings walk in New York, the United States, Jan. 1, 2021. The total number of COVID-19 cases in the United States topped 20 million on Friday, according to the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University.

Workers walk in a corridor of a hospital in New York, the United States, Jan. 1, 2021. The total number of COVID-19 cases in the United States topped 20 million on Friday, according to the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University.

Healthcare workers tranfer a patient into a hospital in New York, the United States, Jan. 1, 2021. The total number of COVID-19 cases in the United States topped 20 million on Friday, according to the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University.

Healthcare workers wheel a patient into a hospital in New York, the United States, Jan. 1, 2021. The total number of COVID-19 cases in the United States topped 20 million on Friday, according to the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University.

A nutcracker sculpture with a mask is seen in New York, the United States, Jan. 1, 2021. The total number of COVID-19 cases in the United States topped 20 million on Friday, according to the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University.

A pedestrian walks past a COVID-19 test site in New York, the United States, Jan. 1, 2021. The total number of COVID-19 cases in the United States topped 20 million on Friday, according to the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University.

Healthcare workers tranfer a patient into a hospital in New York, the United States, Jan. 1, 2021. The total number of COVID-19 cases in the United States topped 20 million on Friday, according to the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University.

A bus driver wearing a face mask works in New York, the United States, Jan. 1, 2021. The total number of COVID-19 cases in the United States topped 20 million on Friday, according to the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University.

People jog near the Lincoln Memorial Reflecting Pool in Washington, D.C., the United States, on Jan. 1, 2021. The confirmed COVID-19 cases in the United States topped 20 million on Friday as the discovery of a highly contagious new virus strain in the country increases pressure to speed up the vaccination process.

A mask is seen at the Lincoln Memorial in Washington, D.C., the United States, on Jan. 1, 2021. The confirmed COVID-19 cases in the United States topped 20 million on Friday as the discovery of a highly contagious new virus strain in the country increases pressure to speed up the vaccination process.

Photo taken on Jan. 1, 2021 shows a road with few cars in Washington, D.C., the United States. The confirmed COVID-19 cases in the United States topped 20 million on Friday as the discovery of a highly contagious new virus strain in the country increases pressure to speed up the vaccination process.

A man wearing a face mask walks on the National Mall in Washington, D.C., the United States, on Jan. 1, 2021. The total number of COVID-19 cases in the United States topped 20 million on Friday, according to the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University. (Photos: Xinhua)

People visit the Lincoln Memorial in Washington, D.C., the United States, on Jan. 1, 2021. The total number of COVID-19 cases in the United States topped 20 million on Friday, according to the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University.

People walk past the Lincoln Memorial Reflecting Pool in Washington, D.C., the United States, on Jan. 1, 2021. The total number of COVID-19 cases in the United States topped 20 million on Friday, according to the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University.