WASHINGTON — Jill Biden, the first lady, tested negative for the coronavirus on Monday after a rebound case had forced her back into isolation, according to the White House.

Dr. Biden first tested positive on Aug. 15 while on vacation with President Biden on Kiawah Island, S.C. Six days later, she received a second consecutive negative test and traveled to Delaware. She had to remain there after she tested positive again last Wednesday in what her office described as a rebound case. A spokeswoman for Dr. Biden said in a statement that the first lady would return to the Washington area on Tuesday.

Mr. Biden also had a rebound case and had to return to isolation. During their bouts with Covid-19, both the president and the first lady took the antiviral treatment Paxlovid, which can reduce the effects of the virus. But researchers are studying whether the medication may also lead to a recurrence of symptoms or positive tests after patients initially test negative.

Experts have stressed that the medication is successful in preventing severe Covid-19 illness and hospitalizations. A study of Paxlovid and another treatment, molnupiravir, that was published in June but has not yet been peer reviewed found that of 13,644 adults, about 5 percent tested positive again within 30 days and 6 percent experienced symptoms again.

The first lady’s office said Dr. Biden’s symptoms did not re-emerge when she tested positive again. She initially experienced mild, coldlike symptoms when she first tested positive. Both Mr. Biden, 79, and Dr. Biden, 71, have been vaccinated twice and received two booster shots.