This post may refer to COVID-19
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Covid: Pfizer says antiviral pill 89% effective in high risk cases
The company stopped clinical trials early because initial results for the drug were so positive.
The drug - Paxlovid - is intended for use soon after symptoms develop in people at high risk of severe disease.
It comes a day after the UK medicines regulator approved a similar treatment from Merck Sharp and Dohme (MSD).
Pfizer says it stopped trials early as the initial results were so positive.
The UK has already ordered 250,000 courses of the new Pfizer treatment along with another 480,000 courses of MSD's molnupiravir pill.
The Pfizer drug, known as a protease inhibitor, is designed to block an enzyme the virus needs in order to multiply. When taken alongside a low dose of another antiviral pill called ritonavir, it stays in the body for longer.
Three pills are taken twice a day for five days.
The combination treatment works slightly differently to the Merck pill which introduces errors into the genetic code of the virus.
Pfizer said it plans to submit interim trial results for its pill to the US medicines regulator, the FDA, as part of the emergency use application it started last month.
The company's chairman and chief executive officer Albert Bourla said the pill had "the potential to save patients' lives, reduce the severity of Covid-19 infections, and eliminate up to nine out of 10 hospitalisations".
Vaccines against Covid-19 are seen as the best way of controlling the pandemic but there is also demand for treatments that can be taken at home, particularly for vulnerable people who become infected.
Interim data from trials of the treatment in 1,219 high-risk patients who had recently been infected with Covid found that 0.8% of those given Paxlovid were hospitalised compared with 7% of patients who were given a placebo or dummy pill.
They were treated within three days of Covid symptoms starting.
Seven patients given the placebo died compared to none in the group given the pill.
When treated within five days of symptoms appearing, 1% given Paxlovid ended up in hospital and none died. This compared to 6.7% of the placebo group being hospitalised and 10 of them dying.
Patients in the trial were elderly or had an underlying health condition which put them at higher risk of serious illness from Covid. They all had mild to moderate symptoms of coronavirus.
Pfizer is also studying the treatment's impact on people at low risk of Covid illness and on those who have already been exposed to the virus by someone in their household.