A single dose of Regeneron Pharma’s antibody cocktail casirivimab and imdevimab (trade name Regen-Cov) reduced the risk of COVID-19 by 81.6% for up to eight months, according to a Phase 3 trial.

Regen-Cov is currently authorized in the US to treat people who are at high risk of serious consequences from COVID-19 infection who are either already infected (non-hospitalized) or in certain post-exposure prophylaxis settings.

The clinical trial jointly run by Regeneron and the National Institute of Allergy and Infectious Diseases (NIAID), assessed the use of a single dose of investigational Regen-Cov (1,200 mg administered via 4 subcutaneous injections) to prevent COVID-19 in uninfected individuals.

According to results of the study of 1,683 patients, compared to placebo (n=842), people who received a single dose of Regen-Cov (n=841) experienced:

  • 81.6% reduced risk of developing COVID-19 during the pre-specified follow-up period, between months 2-8 (7 vs 38 placebo).
  • 81.5% reduced risk of developing COVID-19 at any time during the 8 months after receiving Regen-Cov (20 vs 108 placebo).
  • During the 8-month assessment period, 0 individuals in the Regen-Cov group were hospitalized due to COVID-19, compared to 6 individuals in the placebo group (1 person in the first month; 5 people during months 2-8). There were no deaths due to COVID-19 in any treatment group during the 8-month assessment period, and there were no new safety signals identified for Regen-Cov.

The trial, which was fully enrolled in early 2021, allowed participants to become vaccinated if they wished once the primary efficacy treatment period (month 1) was complete. Vaccination rates during the months 2-8 assessment period were balanced, with 34.5% (n=290) of the Regen-Cov group and 35.2% (n=296) of the placebo group receiving at least 1 COVID-19 vaccine dose by the end of the 8-month assessment period.

Through an innovative trial design, researchers were able to demonstrate the impact of Regen-Cov in high-risk household transmission settings (month 1, both pre- and post-exposure prophylaxis), as well as after the immediate risk of household infection had subsided (months 2-8, pre-exposure prophylaxis), when most infections were presumably acquired in the broader community. During the initial high-risk period related to household transmission, the rate of COVID-19 (in the absence of protection with Regen-Cov) was 13-fold higher than during the subsequent period of ongoing transmission: during month 1, the rate of COVID-19 in the placebo group was 8.3% per month; and during months 2-8 it decreased to 0.6% per month on average.

“Today’s new data demonstrate how a single dose of REGEN-COV can help protect people from COVID-19 for many months after administration,” said Myron S. Cohen, M.D., who leads the monoclonal antibody efforts for the NIH-sponsored COVID Prevention Network (CoVPN) and is Director of the Institute for Global Health & Infectious Diseases at the University of North Carolina at Chapel Hill. “These results demonstrate that REGEN-COV has the potential to provide long-lasting immunity from SARS-CoV-2 infection, a result particularly important to those who do not respond to COVID-19 vaccines including people who are immunocompromised.”