He said he has mild symptoms and will be quarantining for five days.
As the COVID-19 pandemic has swept the globe, more than 5.4 million people have died from the disease worldwide, including over 825,000 Americans, according to real-time data compiled by Johns Hopkins University's Center for Systems Science and Engineering.
About 62% of the population in the United States is fully vaccinated against COVID-19, according to data from the Centers for Disease Control and Prevention.
The Chinese city of Xi'an has 175 of the nation's 209 new COVID-19 cases, health authorities said.
Xi'an is under lockdown, leaving highways and walkways empty.
-ABC News' Christine Theodorou
Vice President Kamala Harris' PCR test came back negative Monday, following close contact with a staffer who tested positive for COVID-19 on Wednesday, according to a White House official.
The staffer was with Harris throughout the day last Tuesday and tested positive the following morning, Harris' spokeswoman, Symone Sanders, said at the time.
Harris has been tested several times since that close contact, always testing negative, officials said.
-ABC News' Molly Nagle
Last week, the CDC reported that omicron had overtaken delta to become the dominant COVID-19 variant in the U.S. However, on Tuesday, the CDC revised its initial estimates to noticeably lower numbers of omicron cases.
Last Monday, the CDC said omicron was estimated to makeup more than 73.2% of new cases as of Dec. 18. Now, the CDC says just 22.5% of new U.S. cases were estimated to be omicron as of Dec. 18.
According to new data from this week, the CDC says omicron is now estimated to account for 58.6% of all new cases, officially making it the dominant strain in the U.S. In the New York and New Jersey region as well as the deep South, omicron is estimated to account for more than 85% of new cases.
A CDC spokeswoman said in a statement, "There was a wide predictive interval posted in last week’s chart, in part because of the speed at which omicron was increasing. We had more data come in from that timeframe and there was a reduced proportion of omicron. It’s important to note that we’re still seeing steady increase in the proportion of omicron."
ABC contributor Dr. John Brownstein, an epidemiologist at Boston Children's Hospital, said, "It may be possible that a large portion of their [the CDC's] sequencing data had come from labs that were specifically looking for s-gene samples, thus falsely elevating the percent omicron in samples."
-ABC News' Arielle Mitropoulos
When New York City students return to school after winter break, they will see PCR testing doubling in every school, Mayor Bill de Blasio announced Tuesday.
Testing will include both vaccinated and unvaccinated students, he said, and teachers and staff will also be able to test.
If a positive case is detected, all students in that class will be given a rapid at-home test. If they are asymptomatic and test negative, they can return the day after their first negative test. Students will then be given a second at-home test within seven days of exposure.
That shifts from the previous plan of sending entire classrooms home to learn online when one or more students test positive.
Schools will still be closed if evidence of major in-school spread.
The city's Department of Education staff is at a 96% vaccination rate, officials said.
-ABC News' Alexandra Faul, Aaron Katersky
