There is “no evidence” that a second course of Pfizer Inc.’s Paxlovid will help COVID-19 patients whose symptoms return after an initial course of the antiviral, a U.S. Food and Drug Administration official said a day after Pfizer executives advocated the idea.

Doctors and virologists have been struggling to understand a number of patient reports of viral rebounds after completion of a five-day course of the COVID-19 treatment. In an interview Tuesday, Pfizer Chief Executive Officer Albert Bourla said doctors could prescribe a second course of treatment to patients who suffer a relapse.

In cases where virus levels rebound, “then you give a second course, like you do with antibiotics, and that’s it,” Bourla said Tuesday. Some physicians have suggested a longer course of Paxlovid might prevent symptoms from returning.

No data support a longer course of treatment or two separate five-day courses to treat or prevent rebound, John Farley, director of the Office of Infectious Diseases in the FDA’s Center for Drug Evaluation and Research, said Wednesday in a post on the agency’s website.

“There is no evidence of benefit at this time” for either repeat dosing or a longer course of treatment, Farley said in the post. “We are continuing to review data from clinical trials and will provide additional information as it becomes available.”

The drugmaker studied a 10-day course of treatment in an early-stage clinical trial, and will keep the public informed on whether it plans to conduct a separate study of the treatment over that duration of time, Pfizer spokesperson Jerica Pitts said.

“As always, patients should consult directly with their physician,” she said.

Analyses of earlier clinical trial data showed that roughly 1 percent to 2 percent of people in an earlier trial had positive polymerase chain reaction, or PCR, tests following a course of Paxlovid course and an initial negative test, Farley said. However, some people who received a placebo also experienced viral rebound, he said.

“It is unclear at this point that this is related to drug treatment,” Farley said in the update. Pfizer has also said that the phenomenon is unlikely to be related to the drug.

The government considers Paxlovid the “preferred therapy for the management of non-hospitalized adults with COVID-19,” the FDA said Wednesday in a statement, citing National Institutes of Health treatment guidelines.

Shares of Pfizer fell 0.4 percent as of 1:26 p.m. in New York.