Pfizer pills effective against omicron, vaccine-maker's studies show

Pharmaceutical company presented results of three separate, unpublished lab studies

Pfizer Inc. said Tuesday that laboratory testing showed its COVID-19 pill Paxlovid appears to be effective against the omicron variant of the coronavirus.

The company said three separate studies demonstrated that the in vitro efficacy of nirmatrelvir, the antiviral treatment's main component, was maintained against the variant of concern.


In an initial study, nirmatrelvir was tested against an enzyme that the coronavirus needs to replicate – called the Mpro – from several variants of concern including omicron in a biochemical assay. 

Pfizer said the results showed in all cases that nirmatrelvir was a "potent inhibitor of its target."

In a second in vitro study, nirmatrelvir was tested against omicron and other variants of concern in an antiviral, cell-based assay. Researchers measured reduction in viral load through polymerase chain reaction (PCR) analysis and found that nirmatrelvir's EC50 was 16 nanomolar for omicron compared with 38 nanomolar for the USA-WA1/2020 variant.

EC50 is a measure of drug potency that shows a concentration that is effective in producing 50% of the maximal response.

A third study in partnership with the Icahn School of Medicine at Mount Sinai in New York City used a SARS-CoV-2-specific immunofluorescence-based assay to detect the virus and measure the in vitro potency of nirmatrelvir and other authorized COVID-19 therapeutics against variants of concern.

Treatments were tested against variants in two cell lines and Pfizer said the results provided confirmation to its findings that nirmatrelvir remains the only orally administrable, authorized compound that has been shown to have low nanomolar in vitro activity against omicron, to date.

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The company wrote that the studies suggest Paxlovid has the potential to maintain plasma concentrations "many-fold times higher" than the amount that is necessary to prevent omicron from replicating in cells. 

Variants of concern can be resistant to treatments that work by binding the spike protein on the SARS-CoV-2 virus. In contrast, Paxlovid works intercellularly and binds the highly conserved Mpro of the virus.

"Following the clinical findings – showing Paxlovid reduced risk of hospitalization or death by nearly 90% compared to placebo for high-risk patients when treated within five days of symptom onset – we are encouraged by these initial laboratory findings," Mikael Dolsten, Pfizer's chief scientific officer, said in a statement. "These data suggest that our oral COVID-19 therapy can be an important and effective tool in our continued battle against this devastating virus and current variants of concern, including the highly transmissible Omicron."


In December, the Food and Drug Administration authorized Paxlovid for emergency use in adults and pediatric patients with positive results of direct SARS-CoV-2 testing and who are at high risk for progression to severe COVID-19, including hospitalization or death. 

The company noted the previous data had indicated Paxlovid maintains in vitro efficacy against earlier and current variants of concern.