Rochelle Walensky, director of the Center for Disease Control and Prevention (CDC), has tested positive for COVID-19 again after completing a round of the coronavirus antiviral treatment Paxlovid.

Walensky tested positive for COVID-19 earlier this month, going into isolation and taking “appropriate action” for her health.

On Monday, the CDC said Walenskt had experienced “mild symptoms” during her infection and eventually tested negative for the virus after completing a round of Paxlovid.

“On Sunday, Dr. Walensky began to develop mild symptoms and has again tested positive. Consistent with CDC guidelines, she is isolating at home and will participate in her planned meetings virtually,” said the agency.

This apparent case of Paxlovid rebound is not surprising. White House chief medical adviser Anthony Fauci and President Biden both tested positive for COVID-19 again following a round of Paxlovid.

A similar phenomenon has been observed in the Merck and Ridgeback coronavirus antiviral molnupiravir. COVID rebound can also occur in cases when antivirals are not administered.

When a rebound in COVID-19 symptoms after antiviral treatments was first observed, it was hypothesized that it was due to an insufficient exposure to the medicine. This hypothesis led some stakeholders to suggest courses of antivirals to last longer than the five days that are currently authorized by the CDC.

However, a small study recently conducted by the National Institute of Health (NIH) found that COVID rebound may instead be caused by a “robust” immune response to “residual viral RNA” within the respiratory tract.

“The results do not support the hypothesis that the five-day course of Paxlovid is too short for the body to develop a strong immune response to SARS-CoV-2,” the NIH said.