Pfizer said Friday that its experimental antiviral pill for COVID-19 cut rates of hospitalization and death by nearly 90%.

The announcement follows a day after the United Kingdom became the first country to give the green light to a pill for coronavirus treatment developed by competitor Merck & Co.

How effective is Pfizer’s COVID-19 pill?

Pfizer’s pill will be given with an older antiviral called ritonavir. The combination treatment will have the brand name Paxlovid and consists of three pills given twice daily.

In a trial study of 775 patients, patients taking the combination treatment saw a 89% reduction in their combined rate of hospitalization or death after a month. This was compared to patients taking a placebo.

Fewer than 1% of patients taking the drug needed to be hospitalized and no one died. In the comparison group, 7% were hospitalized and there were seven deaths.

“We were hoping that we had something extraordinary, but it’s rare that you see great drugs come through with almost 90% efficacy and 100% protection for death,” Mikael Dolsten, Pfizer’s chief scientific officer, said.

Study participants were not vaccinated, showed mild-to-moderate COVID-19, and were considered high risk for hospitalization due to health problems like obesity, diabetes or heart disease. Treatment began within three to five days of initial symptoms, and lasted for five days.

An independent group of medical experts monitoring the trial recommended stopping it early. This is standard procedure when interim results show such a clear benefit.

Pfizer provided few details on side effects but said rates of problems were similar between the groups at about 20%.