Health·CBC Explains

What's happening with antiviral drugs to treat COVID-19?

On Dec. 1, Pfizer submitted its antiviral treatment candidate to Health Canada for authorization.

Pfizer and Merck have both submitted potential treatments to Health Canada for authorization

New antiviral medications announced by Merck and Pfizer appear promising for treating early COVID-19 and keeping patients out of hospital, infectious disease specialists say. But they noted that vaccination remains the best defence against the coronavirus. (Evan Mitsui/CBC)

Since this story was published, Health Canada approved Pfizer's antiviral treatment for COVID-19, Paxlovid, on Jan 17, 2022. It was approved in the U.S. on Dec. 22, 2021. 

The original story can be found below.

Pfizer Canada has submitted its experimental antiviral medication to treat early COVID-19 to Health Canada. 

The submission, received on Dec. 1,  is now "under review," according to Health Canada's COVID-19 drug and vaccine applications database. 

The move comes almost a month after Pfizer announced that its antiviral drug looked so promising that independent experts recommended stopping clinical trials.

Another antiviral pill for the treatment of COVID-19, manufactured by Merck, was submitted to Health Canada on August 13. It is still listed as "under review." 

Experts emphasize that vaccination is still the primary and most effective defence against COVID, but welcome the possibility of an oral medication to save lives and prevent hospitalization among people who become infected. Here's what we know about COVID-19 antivirals so far. 

Have either the Merck or Pfizer antiviral pills been approved in the U.S.?


Merck's pill, called molnupiravir, passed an initial phase of the approval process on Nov. 30, when an expert advisory panel to the U.S. Food and Drug Administration (FDA) recommended the agency approve the drug. But both the FDA and the Centers for Disease Control and Prevention (CDC) still need to officially approve it. 

Pfizer submitted its drug, called Paxlovid, to the FDA on Nov. 16. 

Who are the antiviral pills for?

Both the Merck and Pfizer medications are for adults who are in the early stages of COVID-19 with mild to moderate symptoms and at risk of deteriorating into severe illness and requiring hospitalization. Those at risk could include people who are older or who have underlying conditions, such as obesity, diabetes or heart disease. 

Patients in the clinical trials were unvaccinated. More research is needed, experts say, to find out if the antivirals could help people who are vaccinated but get a breakthrough infection. 

How effective are they?

Pfizer's preliminary results of its clinical trial showed a reduction between 85 and 89 per cent in the combined rate of hospitalization and death among those who got the drug, compared to those who got a placebo pill. 

For patients who started taking the drug within three days of the onset of symptoms, the hospitalization rate was 89 per cent lower. The reduction in hospitalization or death was slightly less (85 per cent) among those who took the pills a day or two later (within five days of symptoms developing).

No one who took the Pfizer antiviral drug died, the company said. 

WATCH | Dr. Zain Chagla says antiviral drugs for COVID-19 look promising:

Pfizer COVID-19 antiviral pill offers huge hope, says specialist

2 years ago
Duration 4:50
Pfizer's experimental antiviral pill for COVID-19 has great potential to drive down mass hospitalizations and reduce death, says infectious diseases specialist Dr. Zain Chagla. 'There is a lot of optimism,' he said. (Bebeto Matthews/The Associated Press)

In its preliminary clinical trial results, Merck reported that its pill reduced the rate of hospitalization by 50 per cent, compared to those who got a placebo pill. That was among patients who started taking the medication within five days of symptoms beginning. 

But in an updated news release issued on Nov. 26, Merck said that data had come in from all participants and the relative risk reduction of hospitalization and death was actually lower, at 30 per cent. 

The updated data showed nine deaths in the placebo group, and one in the group that took molnupiravir. 

Both the Pfizer and Merck trials had been stopped early because the pills appeared to be so effective. Neither study has been peer reviewed. 

How are the pills taken?

Both Merck and Pfizer antiviral pills are taken by mouth twice a day for five days. 

Based on study results so far, they must be taken within five days of developing symptoms to be effective.

How do they work?

Merck's drug works by targeting an enzyme that the coronavirus uses to reproduce itself and inserting errors into its genetic code, disrupting the virus's ability to spread among cells.

Pfizer's drug is a protease inhibitor — a family of drugs used to treat HIV/AIDS and hepatitis C. It works by blocking an enzyme that the virus needs to multiply. Unlike Merck's drug, it does not disrupt the genetic code of the virus. 

The Pfizer pills are taken with an older protease inhibitor called ritonavir, which boosts the medication's levels in the body. Drug combinations like this have worked well in treating HIV/AIDS.   

What are the limitations?

Having an effective treatment that can be taken at home and keeps patients out of hospital is an exciting development, infectious disease specialists say. But the fact they have to be taken within five days of the onset of symptoms is a potential problem. 

That's because during the first five days of COVID-19 infections, people feel relatively well and may just have a runny nose or mild cough and may not seek medical attention, said Dr. Ilan Schwartz, an assistant professor of infectious disease at the University of Alberta. 

"Although that's the mildest part of the disease, that's actually when we need to intervene [with these drugs]," Schwartz said. "So that's going to be a challenge for us going forward."

Ramping up access to care, plus testing, will be critical in order for these drugs to work, Dr. Zain Chagla, an infectious disease specialist at McMaster University, told CBC News Network. 

Isn't there already an antiviral approved to treat COVID-19 in Canada?

Yes — but not one that can be taken in pill form at home.

Remdesivir was authorized by Health Canada in the summer of 2020 for use in hospitalized COVID patients and is administered intravenously. But the World Health Organization has raised questions about its effectiveness on critically ill patients.

Dexamethasone is also used to treat very ill COVID-19 patients, but it's a corticosteroid, not an antiviral. 


Nicole Ireland is a reporter with The Canadian Press.

With files from The Canadian Press, The Associated Press and Reuters

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