The U.S. Congress is experiencing an unprecedented rise in COVID-19 cases, with the seven-day positivity rate at a congressional test site surging to 13 percent from just 1 percent in late November, the Capitol's attending physician said on Monday.

Most coronavirus infections on Capitol Hill have been occurring among the vaccinated, with the Omicron variant representing about 61 percent and the Delta variant 38 percent, based on a limited sample as of December 15, Dr. Brian Monahan told lawmakers and staff in a January 3 letter.