Regen-CoV in early COVID-19

Regen-CoV is the name given to a combination of the following two antibodies:

  • casirivimab
  • imdevimab

These antibodies attack SARS-CoV-2 (the virus that causes COVID-19) and are used as a treatment in some people in the early stages of COVID-19. Regen-CoV is given via intravenous infusion over the course of one hour.

Regen-CoV has been tested in multiple clinical trials. In a phase III trial, participants with mild-to-moderate COVID-19 were randomly assigned to receive a single infusion of different doses of Regen-CoV (1,200 mg or 2,400 mg) or placebo. As the 2,400-mg dose is now the authorized dose in Canada, we will focus on results from participants who received either that dose or placebo.

Researchers found that Regen-CoV reduced the risk of hospitalization or death by 70% compared to placebo. Participants who received Regen-CoV spent less time in hospital (10 days vs. 14 days) compared to participants on placebo. Regen-CoV also reduced the amount of virus in people given the drug. Side effects were not common.

Study details

Participants were enrolled between late September 2020 and mid-January 2021. Their average profile upon study entry was as follows:

  • age – 50 years (14% were 65 years or older)
  • 52% women, 48% men
  • major ethno-racial groups: White – 85%; Hispanic – 35%; Black- 5%; Asian – 4% (numbers do not total 100 due to overlap of categories)
  • BMI (body mass index) – 31 kg/m2
  • common underlying conditions – obesity (58%), cardiovascular disease (36%)


Hospitalization and/or death are commonly assessed outcomes in large studies of treatments for COVID-19. The following proportions of participants were hospitalized and/or died during the study:

  • Regen-CoV – 1.3% (18 of 1,355 participants)
  • placebo – 4.6% (62 of 1,341 participants)

Statistical analysis found that Regen-CoV reduced the risk of hospitalization or death by 71%.

Deaths were distributed as follows:

  • Regen-CoV – 1 person
  • placebo – 3 people

Regen-Cov’s ability to reduce the risk of hospitalization or death became apparent between one and three days after participants received the antibodies.


During a clinical trial, participants can develop adverse events. These require investigation to determine whether they are caused by the underlying disease process, the study drug(s) or something unrelated to the study.

Participants who received placebo were more likely to have adverse events (4%) than people who received Regen-CoV (1.3%). Most of these adverse events were related to COVID-19.

About antibody infusions in general

Antibody-based therapies are routinely used to treat many conditions, including arthritis, some forms of cancer, Crohn’s and colitis, and psoriasis. Doctors and nurses have found that reactions to infusions of these antibodies are relatively common but are generally mild to moderate in people with cancer and inflammatory conditions. General symptoms that can be associated with infusions of antibodies include the following:

  • headache
  • fatigue
  • redness
  • nausea
  • fever
  • pain

These symptoms usually resolve within hours to a day after infusion.

In very rare cases, more serious symptoms, such as anaphylaxis, can occur.

Infusion-related reactions and Regen-CoV

Infusion-related adverse events were generally mild and resolved within a day. In a small number of people, infusion-related adverse events were graded as moderate or of greater severity:

  • Regen-CoV – 2 people
  • placebo – 0 people

These proportions of people represent less than 1% of participants who received Regen-CoV or placebo.

Regen-CoV has been tested in several thousand people in clinical trials. There have been rare cases of hypersensitivity and anaphylactic reactions to the drug.  

Bear in mind

The antibodies in Regen-CoV are potent when used in people with early SARS-CoV-2 infection who have mild-to-moderate symptoms. Regen-CoV was generally well tolerated in this study.

—Sean R. Hosein


  1. Weinreich DM, Sivapalasingam S, Norton T, et al. REGEN-COV antibody combination and outcomes in outpatients with Covid-19. New England Journal of Medicine. 2021; in press.
  2. Austin D. Why do placebos work? Scientists identify key brain pathway. Science. 27 October 2021