By Nandita Bose WASHINGTON (Reuters) -U.S. Vice President Kamala Harris tested positive for COVID-19 on Tuesday but is not exhibiting symptoms, a spokesperson said, at a time when there has been an increase in cases among White House staffers. 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. Harris will isolate and return to the White House when she tests negative, Allen said. Harris came in to the White House on Tuesday but tested positive and left before meeting Biden, a senior admini…