WASHINGTON, United States -- Pfizer said Monday it had begun a middle-to-late stage clinical trial of a pill to stave off Covid in people who are exposed to infection.

Several companies are working on so-called oral antivirals, which would mimic what the drug Tamiflu does for influenza and prevent the disease from progressing to severe.

"We believe that tackling the virus will require effective treatments for people who contract, or have been exposed to, the virus, complementing the impact that vaccines have had," said Mikael Dolsten, the company's chief scientific officer.

Pfizer started developing its drug, called PF-07321332, in March 2020 and is testing it in combination with ritonavir, a repurposed HIV medicine.

The clinical trial will enroll 2,660 adults who will take part at the first signs of Covid infection or at first awareness of exposure.

They will be randomly assigned to receive either a combination of PF-07321332 and ritonavir, or a placebo twice a day for five or ten days.

The objective is to assess the safety and efficacy of the drugs being studied at preventing SARS-CoV-2 infection and symptomatic disease by day 14.

Other companies are also testing existing oral antivirals against Covid -- but Pfizer's is the first specifically designed against the coronavirus.

It is known as a "protease inhibitor" and has been shown in lab testing to jam up the virus' replication machinery.

If it works in real life, it will likely only be effective at the early stages of infection.

By the time Covid progresses to severe disease, the virus has largely stopped replicating and patients suffer from an over-active immune response.