WASHINGTON, Jan 3 (Reuters) - 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% from just 1% 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% and the Delta variant 38%, based on a limited sample as of Dec. 15, Dr. Brian Monahan told lawmakers and staff in a Jan. 3 letter. The surge comes as the number of new COVID-19 cases in the United States has doubled in the last seven days to an average of 418,000 a day, according to a Reuters tally. Monahan noted that breakthrough infections among the vaccinated on Capitol Hill have not led to hospitalizations, serious complications or deaths, a fact he said demonstrated the importance of vaccinations. The U.S. government has been urging vaccinated Americans to get boosters and for the unvaccinated, who are at a much higher risk of getting a severe case of COVID-19 and dying, to be inoculated. About 65% of COVID-19 cases at the Capitol have been symptomatic, according to the letter. In other cases, people testing positive have not shown symptoms. The Senate was due to return on Monday after a year-end holiday break but met for only a brief session due to a snow storm that also forced the Capitol testing site to close early. The attending physician did not call for any change in an existing requirement for masks to be worn in the Capitol whenever others are present, though he advised members and staff to wear medical-grade masks rather than simple cloth ones. He also urged congressional offices, committees and agencies to immediately review operations and adopt "a maximal telework posture" to reduce in-person meetings and in-office activities.