New Delhi: A combination of two antiviral drugs, nirmatrelvir and ritonavir, sold by Pfizer under the brand name Paxlovid, could reduce the risk of progression to severe Covid-19 by 89 per cent, a new clinical trial shows.

The results from the trial were published in The New England Journal of Medicine Wednesday.

A team of researchers from Pfizer conducted a phase 2/3 clinical trial among unvaccinated Covid patients who were symptomatic but not hospitalised, and were at high risk for progression to severe Covid-19.

A total of 2,246 Covid positive patients were included in the trial, of which 1,120 patients received 300 mg of nirmatrelvir plus 100 mg of ritonavir, and 1,126 received placebo (a fake pill) every 12 hours for five days.

The team then evaluated the viral load and Covid-related hospitalisation or death from any cause for 28 days.

Among patients treated within three days after the onset of symptoms, the incidence of Covid-related hospitalisation or death was lower in the nirmatrelvir group than in the placebo group.

The viral load was lower among those who were treated with nirmaltrelvir plus ritonavir than with placebo at Day 5 of treatment, the team reported in the study.

There were 13 reported Covid deaths in the placebo group, while no deaths were reported in the nirmatrelvir group.

The occurrence of adverse events that emerged during the treatment period was similar in both groups.

The team concluded that an oral regimen of nirmatrelvir (300 mg) with ritonavir (100 mg) every 12 hours for five days was linked with an 89.1 per cent relative risk reduction in Covid-related hospitalisation or death by 28 days.

“At the full analysis, relative risk reductions of 88.9 per cent and 87.8 per cent were observed among patients commencing treatment within 3 days and within 5 days after symptom onset, respectively, with 0 deaths occurring in the group that received nirmatrelvir plus ritonavir and 13 deaths occurring in the placebo group,” the team wrote in the study.

Paxlovid got USFDA approval in December

The US Food and Drug Administration issued an emergency use authorisation (EUA) for Paxlovid for the treatment of mild-to-moderate Covid-19 in patients over 12 years of age and weighing at least 40 kilograms in December last year.

Both nirmatrelvir and ritonavir are known as protease inhibitors, which means they bind with specific proteases in the coronavirus and inhibit their functions. Proteases are enzymes that help break down proteins into smaller functional units in viruses, allowing them to carry out the functions required to infect a cell.

Nirmatrelvir, also developed by Pfizer, works on a SARS-CoV-2 protein to stop the virus from replicating, while ritonavir slows down nirmatrelvir’s breakdown, to help it remain in the body for a longer period at higher concentrations.