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Pfizer’s new Covid-19 pill works against Omicron in the laboratory

Pfizer Inc.'s the new Covid-19 pill, Paxlovid, was effective against the Omicron variant in laboratory tests, an encouraging early sign that the drug will be an important tool as the strain spreads.

Pfizer said Tuesday that the drug's main component, nirmatrelvir, worked in three separate laboratory studies. Patients take two tablets of nirmatrelvir with another tablet of another antiviral agent called ritonavir twice daily for five days.

The company released the results in a press release. The research has not been published in a peer-reviewed medical journal.


Pfizer is working to increase the supply of Paxlovid.

Photo: yonhap / Shutterstock

"These data suggest that our oral Covid-19 therapy may be an important and effective tool in our continued fight against this devastating virus and current variants of concern, including the highly transmissible Omicron," said Mikael Dolsten, Pfizer's Chief Scientific Officer.

Last month, the Food and Drug Administration approved the use of Paxlovid in people at high risk for severe Covid-19. Health authorities, doctors and patients say the pill is a valuable addition to the Covid-19 medicine chest because, unlike other available treatments, people who are newly infected can easily take it at home to avoid being hospitalized.

Nevertheless, the emergence of Omicron raised questions about whether Paxlovid, as well as other Covid-19 drugs, would remain effective because the new variant carries many more mutations compared to previous strains, especially to the tip protein protruding from the surface of the virus and plays a crucial role in infecting cells.

Due to the changes, some Covid-19 antibody drugs do not appear to work as well against Omicron.

Many researchers suspected that Paxlovid and another new antiviral pill from Merck & Co. and partner Ridgeback Biotherapeutics LP would work against Omicron because both target the virus' ability to replicate rather than the tip protein.

In a laboratory study, Pfizer researchers tested nirmatrelvir against an enzyme called protease, which the virus needs to generate copies of itself. Nirmatrelvir blocked the protease target in Omicron to the same degree as it blocked the enzyme in the original strain, Pfizer said.

In another laboratory study, the resulting antiviral activity against Omicron was in line with the activity observed by Pfizer in several other variants of concern, including Beta and Delta.

The third study, conducted by researchers at the Icahn School of Medicine at Mount Sinai in New York City with Pfizer, measured the effectiveness of nirmatrelvir against Omicron and other strains and found that a similar concentration of the drug was needed to block infection.

The supply is still limited, although monthly shipments of Paxlovid have begun to reach pharmacies and hospitals for delivery. Pfizer will only finish delivering in June at least half of the 20 million treatment courses purchased by the United States

The FDA has approved Merck's new Covid-19 therapy, molnupiravir, the latest antiviral drug that adults can take at home to avoid serious illness. WSJ's Daniela Hernandez explains the science behind the new drug. Photo: Merck

Pfizer scales up production capacity, but the end-to-end process, from raw ingredients to finished dosages, can take about nine months, according to the company.

The drugmaker expects to produce around seven million courses globally this quarter on its way to making 120 million courses by the end of the year, CEO Albert Bourla said during a recent investor conference.

"Any decent manufacturer of medicine can make it, but the chemistry, it's complicated," said Mr. Bourla.

Officials at the Biden administration said last week that they were working with Pfizer to shave about seven months from the drug's clinical trial, so the pill is available months before the original timeline.

Write to Jared S. Hopkins at

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