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A new antiviral medicine for covid-19, given as an injection within a week of developing symptoms, halves the number of people needing hospital treatment, a large trial has found.

The result is promising because the medicine seems to be as effective in people who caught the omicron variant as in those who got previous variants of the virus, as well as in people who are vaccinated.

The treatment, a compound called pegylated interferon lambda, was tested in a randomised trial of nearly 2000 people living in either Canada or Brazil with covid-19. They were aged 50 or over, or medically vulnerable in some way.

Of those who received the treatment, 2.7 per cent later needed to visit a hospital emergency department or were admitted to hospital. The other participants were given a placebo and 5.6 per cent had to go to hospital. The number of side effects was similar in treatment and placebo groups.

Some other antiviral medicines have been largely abandoned since the advent of omicron because they are less effective against the strain. For instance, a treatment called sotrovimab is no longer widely used because it is an antibody directed against the virus鈥檚 spike protein, but omicron has mutations that mean the drug can no longer bind to this protein so well.

Lambda interferon is a natural compound made by human cells that signal to the immune system that viruses are present and therefore may have broad activity against a range of viruses, says at Stanford University, who was involved in the trial. 鈥淚 hope that the next study will test it in people with any acute respiratory symptoms.鈥�

The pegylated version of lambda interferon is modified to make it more soluble and less likely to trigger antibodies made against it. The compound was already being developed by US firm as a treatment against a virus that causes the liver disease delta hepatitis.

More than eight in 10 participants in this trial had been vaccinated against covid-19. Some other antivirals gave impressive results in initial studies, but when they were tested in vaccinated people, their effectiveness was lower because people who are vaccinated are less likely to get badly ill.

For instance, an early trial found molnupiravir reduced hospital admissions and deaths by 30 per cent. But a later trial testing it against omicron in vaccinated people found it didn鈥檛 reduce either of these outcomes, although it did .

Another current treatment called Paxlovid has been found to reduce hospital admissions and deaths by nearly 90 per cent 鈥� but results from a UK trial testing it in vaccinated people with omicron are still awaited.

Also, not everyone can take Paxlovid because it can interact with other medicines people may be taking, says Janet Scott at the University of Glasgow, UK. 鈥淚t鈥檚 exciting to have a broad range of tools available to use because different drugs will suit different people,鈥� she says.

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