Photo taken on March 20, 2020 shows the U.S. national flag in Washington D.C., the United States. The number of COVID-19 cases in the United States topped 16,000 by 3:30 p.m. local time Friday (1930 GMT), according to the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University. (Photos: Xinhua)

Photo taken on March 20, 2020 shows an empty street in Washington D.C., the United States. The number of COVID-19 cases in the United States topped 16,000 by 3:30 p.m. local time Friday (1930 GMT), according to the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University.

Photo taken on March 20, 2020 shows the White House in Washington D.C., the United States. The number of COVID-19 cases in the United States topped 16,000 by 3:30 p.m. local time Friday (1930 GMT), according to the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University.

A man walks with the Washington Monument in the background in Washington D.C., the United States, on March 20, 2020. The number of COVID-19 cases in the United States topped 16,000 by 3:30 p.m. local time Friday (1930 GMT), according to the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University.