Pfizer says antiviral pill cuts risk of severe Covid-19 by 89%

A trial of Pfizer Inc's experimental antiviral pill for Covid-19 was stopped early after the drug was shown to cut by 89 per cent the chances of hospitalization or death for adults at risk of developing severe disease, the company said on Friday.

The results appear to surpass those seen with Merck & Co Inc's pill, molnupiravir, which was shown last month to halve the likelihood of dying or being hospitalized for Covid-19 patients also at high risk of serious illness.

Full trial data is not yet available from either company.

Pfizer shares surged 13 per cent to $49.47, while those of Merck fell 6 per cent to $84.69.

Pfizer said it plans to submit interim trial results for its pill, which is given in combination with an older antiviral called ritonavir, to the US Food and Drug Administration as part of the emergency use application it opened in October.

The combination treatment, which will have the brand name Paxlovid, consists of three pills given twice daily.

The planned analysis of 1,219 patients in Pfizer's study looked at hospitalizations or deaths among people diagnosed with mild to moderate Covid-19 with at least one risk factor for developing severe disease, such as obesity or older age.

It found that 0.8 per cent of those given Pfizer's drug within three days of symptom onset were hospitalized and none had died by 28 days after treatment. That compared with a hospitalisation rate of 7 per cent for placebo patients. There were also seven deaths in the placebo group.

Rates were similar for patients treated within five days of symptoms - 1 per cent of the treatment group was hospitalized, compared with 6.7 per cent for the placebo group, which included 10 deaths.

Antivirals need to be given as early as possible, before an infection takes hold, in order to be most effective. Merck tested its drug within five days of symptom onset.

"We saw that we did have high efficacy, even if it was five days after a patient has been treated ... people might wait a couple of days before getting a test or something, and this means that we have time to treat people and really provide a benefit from a public health perspective," Annaliesa Anderson, head of the Pfizer program, told Reuters.

The company did not detail side effects of the treatment, but said adverse events happened in about 20 per cent of both treatment and placebo patients.

Pfizer is also studying whether its pill could be used by people without risk factors for serious Covid-19 as well as to prevent coronavirus infection in people exposed to the virus.