Pfizer (NYSE:PFE) announced that its COVID-19 antiviral Paxlovid failed to reach statistical significance in reducing risk after patients were exposed to the SARS-CoV-2 virus.

The primary endpoint of the Phase 2/3 EPIC-PEP (Evaluation of Protease Inhibition for COVID-19 in Post-Exposure Prophylaxis) study was to reduce the risk of confirmed and symptomatic COVID-19 in adults after exposure to a household contact.

Paxlovid (nirmatrelvir [PF-07321332] tablets and ritonavir tablets) led to risk reductions of 32% at day five of therapy. That figure climbed to 37% at day ten of treatment.

The study was completed during the omicron wave.

The emergency use authorization for the drug calls for the drug to be administered twice daily for five days.

Paxlovid appeared to be well tolerated across the EPIC-PEP study and the related EPIC-HR and EPIC-SR trials. In addition, Pfizer noted that the post-market safety data were also encouraging.

The study recruited 2,957 adults. Patients were randomized 1:1:1 to receive Paxlovid for five days, ten days, or a placebo.

“While we are disappointed in the outcome of this particular study, these results do not impact the strong efficacy and safety data we’ve observed in our earlier trial for the treatment of COVID-19 patients at high risk of developing severe illness, and we are pleased to see the growing global use of PAXLOVID in that population,” said Pfizer CEO Dr. Albert Bourla, in a news release.

According to Bloomberg, U.S. government scientists plan on studying Paxlovid after anecdotal evidence has surfaced showing that SARS-CoV-2 counts have rebounded in some patients who finished a course of the drug.

Some patients may benefit from a longer course of treatment, said Dr. Clifford Lane, NIAID deputy director. “There’s two things that suppress the virus: the drug and the host immune response,” Lane told Bloomberg. “If you stop the drug before the host immune response has had a chance to kick in, you may see the virus come back.”

Pfizer has stressed that such anecdotal rebounds are unlikely related to Paxlovid therapy.