Paxlovid: How does new Covid pill work? And will it be impacted by Omicron?

Antiviral pill is nearly 90 per cent effective in cutting risk of hospitalisation and death in vulnerable Covid patients

Samuel Lovett
Science Correspondent
Friday 31 December 2021 12:51 GMT

A second antiviral drug that significantly reduces the risk of hospitalisation and death from Covid-19 has been approved for use by the UK’s medicines regulator.

Paxlovid, which is consumed orally as a pill, will be given to people with mild to moderate infection who are at an increased risk of developing severe disease.

This follows a “rigorous review” of the drug’s safety, quality and effectiveness by the Medicines and Healthcare products Regulatory Agency.

How does it work?

Paxlovid combines two active ingredients, PF-07321332 and ritonavir, that works by inhibits the break down of proteins required for viral replication.This prevents SARS-Cov-2 from multiplying after an infection, keeping virus levels low and helping the immune system to overcome the pathogen.

Ritonavir slows the breakdown of the second ingredient PF-07321332 in the body, thereby increasing the drug’s effectiveness. It has been used alongside some HIV medicines for many years to ‘boost’ their activity

How is it taken?

The two active substances of Paxlovid come as separate tablets that are taken together, twice a day for five days.

It will be made available to people who have recently tested positive for Covid and are considered at-risk, including the over-60s and those with diabetes or heart disease.

How effective is it?

Very. Clinical trial data published by Pfizer, the manufacturer of the drug, show it cuts rates of hospitalisation and death by nearly 90 per cent among vulnerable patients.

Pfizer's study looked at hospitalisations or deaths among 775 unvaccinated people diagnosed with mild to moderate Covid-19. Patients had at least one risk factor for developing severe disease, such as obesity or older age. Treatment began within three to five days of the development of symptoms, and lasted for five days.

It found that 0.8 per cent of those given Pfizer's drug within three days of symptom onset were hospitalised. None had died by 28 days after treatment. This compared with a hospitalisation rate of seven per cent for those patients given a placebo. There were also seven deaths within this group.

Rates were similar for patients treated within five days of symptoms: one per cent of the treatment group was hospitalised, compared with 6.7 per cent for the placebo group, which included 10 deaths.

Paxlovid is most effective when taken during the early stages of infection and so the MHRA recommends its use as soon as possible and within five days of the start of symptoms.

Will the effectiveness of the pill be impacted by Omicron?

At this stage, the government has insisted it is too early to know whether Omicron has any impact on Paxlovid’s effectiveness, but the MHRA is proactively working with Pfizer to establish this.

However, scientists are optimistic that Paxlovid will continue to work.

Testing suggests that Omicron has not acquired any mutations which, “would signal any resistance or change in susceptibility of the virus” to the pill, according to Professor Wendy Barclay, a virologist at Imperial College London.

This is because the vast majority of Omicron’s mutations are concentrated in its spike protein - the part of the virus responsible for gaining entry to human cells - and are not targeted by Paxlovid, which instead interferes with an enzyme linked to the replication of SARS-Cov-2.

How many supplies has the UK ordered?

A total of 2.75 million doses of the pill are expected to be delivered to the UK throughout 2022, Pfizer has said.

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