REGEN-COV was one of the first COVID-19 treatments built from lab-made copies of the antibodies a body makes to fight infection. It was given early in illness or after an exposure, and against the virus versions circulating in 2020 and 2021 it worked well, keeping many high-risk people out of the hospital. The same precision that made it effective became its weakness once the virus changed shape, and by early 2022 it was no longer allowed for use anywhere in the U.S.1U.S. Food and Drug Administration. Frequently Asked Questions on the Emergency Use Authorization of REGEN-COV (Casirivimab and Imdevimab)
What Was Inside REGEN-COV
REGEN-COV combined two monoclonal antibodies, casirivimab and imdevimab, given together in one dose.2U.S. Food and Drug Administration. FDA authorizes REGEN-COV mAb for prevention for COVID-19 A monoclonal antibody is a lab-made protein that copies the immune system’s way of grabbing a harmful virus.3U.S. Food and Drug Administration. FDA authorizes REGEN-COV mAb for prevention for COVID-19 The target was the spike protein, the knobbed protein studding the surface of the coronavirus.
The two antibodies were found by screening ones made by specially engineered mice with human-like immune genes and ones from the blood of people who had recovered from COVID-19.4PubMed Central. Human Monoclonal Antibodies: On the Menu of Targeted Therapeutics Against COVID-19 The mice, when exposed to the spike, make fully human-style antibodies, which gave scientists a large, controllable library to search. Screening meant testing many candidates and keeping those that gripped the spike tightly and blocked its attachment, so the final two were winners from a much larger pool. The chosen pair went from selection to the first human trials in just 56 days.5PubMed Central. Speed and need: twin development challenges in rapid response for a SARS-CoV-2 antibody cocktail
A vaccine teaches your body to make its own antibodies over weeks. REGEN-COV handed you ready-made ones that started working right away. The protection was borrowed, so it faded as the body cleared the antibodies, which is why it suited people already sick or just exposed.
How the Antibodies Blocked the Virus
The coronavirus enters cells by using the tip of its spike to grab a receptor on human cells. This receptor sits on the surface of cells in the nose, throat and lungs and normally does other jobs in the body; the virus simply borrows it as a doorway. Once the tip attaches, the virus can fuse with the cell and release its genetic material inside. That is the step the antibodies aimed to prevent.
Each antibody clamped very tightly onto a different, non-overlapping spot on that tip.6PubMed Central. Casirivimab/Imdevimab: First Approval In lab tests each antibody alone stopped at least 95% of spike tips from attaching to the receptor.7PubMed Central. Casirivimab/Imdevimab: First Approval A virus stuck outside cells cannot copy itself.
An antibody that neutralizes a virus physically prevents it from infecting a cell, rather than merely tagging it for the immune system to clear later. The 95% figure came from controlled lab tests of attachment, so it shows the mechanism works but is not the same as the benefit seen in patients.
Two antibodies were used because viruses given a single antibody can quickly mutate the one spot it targets and slip past it.8PubMed Central. Antibody cocktail to SARS-CoV-2 spike protein prevents rapid mutational escape seen with individual antibodies To escape a two-antibody cocktail, the virus must change two separate spots at once, which is unlikely.9PubMed Central. Antibody cocktail to SARS-CoV-2 spike protein prevents rapid mutational escape seen with individual antibodies Because the antibodies attached to non-overlapping spots, they did not compete for room, and both could bind the same spike tip at once.
In lab tests, a change at one spot only slightly weakened the virus’s grip on cells, but changes at both spots crippled it.10PubMed Central. Molecular determinants and mechanism for antibody cocktail preventing SARS-CoV-2 escape Before Omicron, no natural variant was seen carrying changes at both antibodies’ spots.11PubMed Central. Molecular determinants and mechanism for antibody cocktail preventing SARS-CoV-2 escape
Antibodies stop new cells from being infected but cannot repair damaged ones, and late in illness much of the harm comes from the body’s own inflammation. That is why the treatment was offered to people within days of symptoms or exposure.
How Well It Worked Against Earlier Variants
In a large trial of high-risk people with early COVID-19, 1.3% of those given REGEN-COV were hospitalized or died, versus 4.6% of those given a dummy treatment.12PubMed. REGEN-COV Antibody Combination and Outcomes in Outpatients with Covid-19 That cut the risk by about 71%.13PubMed. REGEN-COV Antibody Combination and Outcomes in Outpatients with Covid-19 A half-size dose worked about as well, cutting the risk by about 70%.14PubMed. REGEN-COV Antibody Combination and Outcomes in Outpatients with Covid-19 In plain numbers, roughly 46 in 1,000 untreated high-risk patients ended up hospitalized or dead, against about 13 in 1,000 treated patients.
Some volunteers got an inactive infusion that looked the same, so any difference could be credited to the antibodies rather than to chance or hope. Participants had early COVID-19 and were at higher risk, so the results apply to that kind of patient and not necessarily to healthy young people.
In everyday use outside trials, treated patients had about 56% less risk of hospitalization and about 93% less risk of dying from COVID-19 within 28 days.15PubMed Central. Effectiveness of REGEN-COV antibody combination in preventing severe COVID-19 outcomes People 60 and older got similar protection, with about 94% lower risk of death.16PubMed. Effectiveness of REGEN-COV antibody combination in preventing severe COVID-19 outcomes Outside trials, patients are more varied and not randomly assigned, so researchers compare treated and untreated people and adjust for differences. The outcomes were also different, so these figures shouldn’t be set directly against the trial percentages.
Eligibility followed risk: older adults and people with diabetes, obesity or weak immunity. It was given soon after a positive test, not to hospitalized patients on oxygen, and it was also offered to exposed people unlikely to respond well to vaccines.
It could also prevent illness. Among people living with someone infected, one shot under the skin cut symptomatic infections by about 81% over a month.17PubMed. Subcutaneous REGEN-COV Antibody Combination to Prevent Covid-19 Of those given it, 1.5% got sick with symptoms, against 7.8% without it.18PubMed. Subcutaneous REGEN-COV Antibody Combination to Prevent Covid-19 From week 2 to week 4 the protection was even stronger, about 93%.19PubMed. Subcutaneous REGEN-COV Antibody Combination to Prevent Covid-19
Drip or Shot
REGEN-COV was first given by intravenous drip and later also as shots under the skin.20PubMed Central. Subcutaneous REGEN-COV Antibody Combination in Early Asymptomatic SARS-CoV-2 Infection: A Randomized Clinical Trial Shots reduced the virus in the body about as much as the drip did.21PubMed Central. Subcutaneous REGEN-COV Antibody Combination in Early Asymptomatic SARS-CoV-2 Infection: A Randomized Clinical Trial
The drip took a clinic chair, roughly an hour, and time to watch for reactions afterward. The shot version used several injections, often in the belly or thigh. An IV needs a nurse and a line, which limited how many patients a site could treat in a day, so shots let busy clinics treat more people during surges.
In a large outpatient center, shots showed no clear rise in deaths, intensive care stays or ventilator use compared with drips.22PubMed Central. Association of Subcutaneous or Intravenous Administration of Casirivimab and Imdevimab Monoclonal Antibodies With Clinical Outcomes in Adults With COVID-19 The same data left open a small possible rise in overall hospital stays with shots.23PubMed Central. Association of Subcutaneous or Intravenous Administration of Casirivimab and Imdevimab Monoclonal Antibodies With Clinical Outcomes in Adults With COVID-19 Another real-world comparison found no increased hospitalization or intensive care with shots.24PubMed. Noninferiority of Subcutaneous Versus Intravenous Casirivimab/Imdevimab for Outpatient Treatment of SARS-CoV-2 in a Real-World Setting
Studies that look back at records cannot fully rule out differences between people who got drips and those who got shots. Even so, the overall evidence suggested shots worked about as well.
Why the FDA Stopped Its Use
Omicron carried 13 to 15 changes on the spike tip, while earlier variants had one to six.25PubMed Central. Omicron variant receptor-binding domain phylogenetics and molecular dynamics Several of those changes sat on or beside spots known to let the virus slip past REGEN-COV’s antibodies.26PubMed Central. Omicron: A Heavily Mutated SARS-CoV-2 Variant Exhibits Stronger Binding to ACE2 and Potently Escapes Approved COVID-19 Therapeutic Antibodies At least two physically push imdevimab’s grip loose, one by bumping into it and one by repelling it electrically.27PubMed Central. Structure and dynamics of the interaction of Delta and Omicron BA.1 SARS-CoV-2 variants with REGN10987 Fab reveal mechanism of antibody action Together with changes affecting casirivimab, they undercut the whole pair. The result was that Omicron strongly weakened both antibodies’ ability to attach, while barely affecting how well the virus grabbed cells.28PubMed Central. Omicron’s binding to sotrovimab, casirivimab, imdevimab, CR3022, and sera from previously infected or vaccinated individuals
An antibody grips a precise three-dimensional shape, so even one swapped building block can loosen the fit. A drug made from one fixed antibody cannot adapt, while a vaccinated person’s immune system makes many varied antibodies. So the drug could lose power once the shape changed, even though it had worked well before.
Omicron was fully or partly resistant to all nine antibody treatments tested in one lab comparison.29PubMed. Considerable escape of SARS-CoV-2 Omicron to antibody neutralization Casirivimab showed no activity against the early Omicron strains BA.1 and BA.2, and imdevimab kept only a trace against BA.2.30PubMed Central. Different efficacies of neutralizing antibodies and antiviral drugs on SARS-CoV-2 Omicron subvariants, BA.1 and BA.2 Later Omicron strains through 2024 stayed highly resistant to both.31PubMed Central. Impact of SARS-CoV-2 variant mutations on susceptibility to monoclonal antibodies and antiviral drugs: a non-systematic review, April 2022 to October 2024
REGEN-COV had been used under an emergency use authorization, a temporary permission the FDA can grant in a crisis and revoke.32U.S. Food and Drug Administration. Frequently Asked Questions on the Emergency Use Authorization of REGEN-COV (Casirivimab and Imdevimab) On January 24, 2022, with Omicron dominant, the FDA ended its use in every U.S. region.33U.S. Food and Drug Administration. Frequently Asked Questions on the Emergency Use Authorization of REGEN-COV (Casirivimab and Imdevimab) Doctors had no quick test to tell whether a patient had a rare older variant the drug could still stop.34eLife. Exploratory data on the clinical efficacy of monoclonal antibodies against SARS-CoV-2 Omicron variant of concern Italy’s drug agency took the same step, following U.S. treatment guidelines.35eLife. Exploratory data on the clinical efficacy of monoclonal antibodies against SARS-CoV-2 Omicron variant of concern
Keeping it for some patients made little sense: giving a drug that probably won’t work delays treatment that will, and wasted doses cost supply and clinic time. The decision was about the variant, not a safety problem with the drug.
What Replaced It
The first two antibody combinations, including REGEN-COV, were largely inactive against BA.1 and BA.2.36PubMed Central. Susceptibility of SARS-CoV-2 Omicron Variants to Therapeutic Monoclonal Antibodies: Systematic Review and Meta-analysis Sotrovimab lost about 4-fold strength against BA.1 and 17-fold against BA.2.37PubMed Central. Susceptibility of SARS-CoV-2 Omicron Variants to Therapeutic Monoclonal Antibodies: Systematic Review and Meta-analysis Bebtelovimab showed no loss of strength against BA.1 or BA.2.38PubMed Central. Susceptibility of SARS-CoV-2 Omicron Variants to Therapeutic Monoclonal Antibodies: Systematic Review and Meta-analysis Sotrovimab kept partial activity, so it was used for a time when REGEN-COV could not be, and bebtelovimab became a main option while those strains dominated. Later strains weakened sotrovimab and the preventive combination tixagevimab and cilgavimab as well.39PubMed Central. Impact of SARS-CoV-2 variant mutations on susceptibility to monoclonal antibodies and antiviral drugs: a non-systematic review, April 2022 to October 2024 Each was useful only until the virus changed again, and drugs aimed at the spike tip run into the part of the virus that changes most.
Newer work looks at antibodies that target spots the virus can’t easily change and still neutralize many variants.40PubMed Central. Broadly neutralizing antibodies overcome SARS-CoV-2 Omicron antigenic shift These broad antibodies tend to be weaker than ones aimed at the spike tip.41PubMed Central. Broadly neutralizing antibodies target the coronavirus fusion peptide Some recovered patients make antibodies that mimic the cell receptor, which the virus can’t escape without hurting itself.42PubMed Central. Broadly neutralizing SARS-CoV-2 antibodies through epitope-based selection from convalescent patients
If you’re at high risk now, ask your doctor about antiviral pills within days of symptoms, and test promptly, because early treatment works best. Staying current on vaccines helps too, since they are updated for new strains.
Did REGEN-COV Affect Later Vaccines
When REGEN-COV was still in the body at vaccination, the virus-blocking antibodies the vaccine made were lower.43PubMed. Monoclonal antibodies against the spike protein alter the endogenous humoral response to SARS-CoV-2 vaccination and infection The lab-made antibodies covered the spots the immune system would otherwise learn to target, so it had less chance to see and learn them.44PubMed. Monoclonal antibodies against the spike protein alter the endogenous humoral response to SARS-CoV-2 vaccination and infection
Overall immune responses to the spike, including T cells, were not dampened.45PubMed. Monoclonal antibodies against the spike protein alter the endogenous humoral response to SARS-CoV-2 vaccination and infection T cells recognize infected cells and help limit severe illness, separately from blocking antibodies. Months later, people who had received antibody treatment still had lower blocking antibodies, but T-cell responses were unchanged.46PubMed Central. Impact of COVID-19 Monoclonal Antibody Therapy on Subsequent Vaccine-Elicited SARS-CoV-2 Immune Responses An extra booster reversed the drop.47PubMed. Monoclonal antibodies against the spike protein alter the endogenous humoral response to SARS-CoV-2 vaccination and infection
This is not a reason to skip vaccines. Tell your doctor you had antibody treatment if timing questions come up.