Paxlovid’ is an oral antiviral drug that reduces the ability of SARS-CoV-2, the virus that causes Covid-19, to multiply in the body. It is a combination of two drugs, Ritonavir, an antiretroviral used against HIV, and PF-07321332, an experimental drug.

The indication would be to treat adults with Covid-19 who do not require supplemental oxygen, but have an increased risk of progressing to severe disease.

The two drugs that make up Paxlovid are protease inhibitors. The active ingredient PF-07321332 blocks the activity of an enzyme needed for the virus to multiply. Ritonavir slows the breakdown of PF-07321332, allowing it to remain in the body longer at levels that affect the virus.

‘Paxlovid’ should be administered as soon as possible after diagnosis of Covid-19 and within 5 days of symptom onset. Each course of Paxlovid consists of 30 pills administered over 5 days. The two active ingredients of the drug, PF-07321332 and ritonavir, which are available as separate tablets, must be taken together every 12 hours. Efficacy decreases as the days pass from the onset of symptoms. If started after three days, it reaches 89%. If started after the fifth day, it is 88%.

The EMA recommendation is based on interim results from a study in non-hospitalized, unvaccinated patients with symptomatic disease and at least one underlying condition that put them at risk for severe Covid-19.

These data showed that ‘Paxlovid’ reduced the risk of hospitalization and death when treatment was initiated within 5 days of symptom onset. Approximately 1 percent of patients (6 of 607) who took this antiviral within 5 days of symptom onset were hospitalized within 28 days of starting treatment, compared with 6.7 percent of patients (41 of 612) given placebo (a dummy treatment); none of the patients in the ‘Paxlovid’ group died, compared with 10 patients in the placebo group.

A laboratory analysis by Pfizer Pharmaceuticals indicates that the antiviral remains highly potent with the Omicron variant, which is expected to be the predominant variant globally after its breakthrough in South Africa and Europe. Laboratory experiments indicated that the drug targets a key protein of the new variant.

In terms of safety, the most common side effects reported during treatment and up to 34 days after the last dose were dysgeusia (taste disturbance), diarrhea and vomiting.

The EMA specified that ‘Paxlovid’ should not be used with some other drugs, either because their action may cause a harmful increase in blood levels, or because, conversely, some drugs may reduce the activity of the antiviral itself. ‘Paxlovid’ should also not be used in patients with severely reduced renal or hepatic function.

It is also not recommended during pregnancy and in people who may become pregnant and are not using contraception. Breast-feeding should be discontinued during treatment. These recommendations are due to the fact that laboratory studies in animals suggest that high doses of ‘Paxlovid’ may affect the growth of the fetus.

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