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People wait in line at a COVID assessment centre at Women's College Hospital on Sept. 23, 2020.Nathan Denette/The Canadian Press

An antiviral drug that is administered in a single shot could be an effective addition to the available treatments for COVID-19, particularly for high-risk patients, a new study has found.

Results of the phase 3 clinical trial, published in the New England Journal of Medicine on Wednesday, showed that COVID patients who received the drug, called pegylated interferon lambda, were half as likely to be admitted to hospital or to visit an emergency department than those given a placebo.

“It’s easy to take, and the advantage to it is it’s sort of like one and done. You get your positive test, you get this injection. You don’t need to remember to take pills multiple times a day,” said Jordan Feld, a physician and senior scientist with Toronto’s University Health Network, who co-authored the study.

Unlike Paxlovid, which is taken as a combination of tablets, interferon lambda has very few drug interactions, Dr. Feld said. So for people who can’t take Paxlovid while they’re on other medications, or who have trouble taking pills, “this would be a particularly good option,” he said.

While there are also a number of other treatments for COVID, there are some limitations to using them. Remdesivir is given intravenously, and thus available only to patients who can access clinics that administer it. And some of the monoclonal antibodies used earlier in the pandemic have become “effectively useless” as the virus has changed, Dr. Feld said.

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Since the interferon works by stimulating the body’s own immune response, it is unlikely that any new variants would be resistant to it, he said. And because it is a single-dose injection, it can easily be given by a doctor, nurse, pharmacist or even by patients themselves.

Interferons are small proteins that the body naturally makes to defend itself from infections. They attach to receptors on cells that trigger a wide range of responses to fight off viruses.

While there are different types, Dr. Feld and his colleagues focused on interferon lambda, a Type III interferon. Unlike interferon alpha, a Type I interferon that has been used for treating hepatitis C and attaches to a receptor found on cells throughout the body, interferon lambda attaches to receptors on cells that exist only in limited places in the body, particularly the lung, liver and the intestine.

“So now we’re getting an interferon with all of its antiviral properties, but it’s only active where we want it to be active,” Dr. Feld said, explaining this avoids the risk of triggering a cytokine storm and other side effects. The study participants reported few or no side effects.

The trial involved more than 1,900 participants in Brazil and Canada, who had at least one health factor that put them at high risk of severe COVID. This included being age 50 or older, having diabetes, cardiovascular disease or obesity. More than 80 per cent of them had been vaccinated for COVID.

Within seven days of their first COVID symptoms, 933 patients were assigned to receive interferon lambda, and 1,018 were given a placebo. A total of 25 patients in the interferon group required hospital admission or an emergency visit, compared with 57 in the placebo group.

An earlier study, led by Dr. Feld, showed that interferon lambda shortened the duration of a COVID infection.

Tom Hobman, a virologist and cell biologist at the University of Alberta who was not involved in the research, said the timing of administration is very important. Interferon is only effective when given early or prophylactically, that is, as a preventative measure, Dr. Hobman said. In animal models, if it’s given late, it can make things worse by exacerbating inflammation, he said.

Jorg Fritz, associate professor in the department of microbiology and immunology at McGill University, who was also not involved in the study, said the findings are promising. He emphasized the need to keep researching new COVID treatments.

“Coronaviruses will always be a challenge to humanity and other animals as well. So having a large arsenal of protective gear in our closets will be very important,” Dr. Fritz said.