Harris has not been in close contact with President Joe Biden or first lady Jill Biden due to their travel schedules, her press secretary, Kirsten Allen, said on Tuesday.
Harris would isolate and return to the White House when she tested negative, Allen said.
After consultation with her physicians, Harris was prescribed and has taken antiviral drug Paxlovid, Allen said.
Harris went in to the White House on Tuesday but tested positive and left before meeting Biden, a senior administration official said.
Infections have risen in some parts of the United States due to the highly transmissible Omicron BA.2 subvariant, even as cases are falling overall, according to public health officials.
In recent weeks, second gentleman Doug Emhoff, Harris' communications director Jamal Simmons, White House press secretary Jen Psaki and deputy press secretary Karine Jean-Pierre disclosed positive tests.
Other high-ranking officials to test positive this month have included US House Speaker Nancy Pelosi, next in the line of succession to the presidency after Harris, Attorney-General Merrick Garland and Commerce Secretary Gina Raimondo.
Biden and Harris are fully vaccinated against COVID-19 and have received two booster shots.
They have in recent weeks mingled with unmasked guests at the White House, including an event attended by 200 guests with former president Barack Obama to talk about Democrats' signature healthcare law.
