Dr. Biden is experiencing mild symptoms, Elizabeth Alexander, her communications director, said in a statement. The first lady has been prescribed Paxlovid, an antiviral drug that reduces the effects of the infection, and will not return to the White House until she receives two consecutive negative tests, Ms. Alexander said. “The first lady began experiencing cold-like symptoms late at night,” Ms. Alexander said in the statement. “It was again negative in a rapid antigen test, but the PCR test was positive. The first lady is double vaccinated, twice boosted and has only mild symptoms.” A statement sent to reporters said Mr. Biden tested negative on Tuesday morning. He returned to the White House to sign the climate and tax bill in the afternoon and continued to Wilmington, Del., for the rest of the week as planned. “Consistent with CDC guidance, because she is a close contact with the first lady, she will wear a mask for 10 days when indoors and around others,” the White House statement said. “We will also increase the rate of testing the president and report those results.”