Monoclonal antibodies are proteins made by cloning a single B cell (hence “mono clonal”) into cultures that synthesize the antibody in sufficient quantities for research and, eventually, formulation into powerful medicine. Two Nobel Prizes have been awarded for monoclonal antibodies: to Jerne, Köhler, and Milstein in 1984 for discovering monoclonal antibodies, and to Allison and Honjo in 2018 for applying them to cancer therapy. Last year, noted Harvard biologist William Haseltine predicted that monoclonal antibodies would be the first therapy specifically developed for SARS-CoV-2 (

He was right. On November 9, 2020, the U.S. FDA issued an emergency use authorization (EUA) for the investigational monoclonal antibody bamlanivimab (LY-CoV555, Eli Lilly) to treat mild-to-moderate cases of COVID-19 in adult and pediatric patients. Bamlanivimab is an antibody directed against the SARS-CoV-2 spike protein. Monoclonal antibodies tend to have unpronounceable names, but the “mab” at the end of the name stands for “monoclonal antibody.”

Bamlanivimab monotherapy

A preplanned interim analysis of the BLAZE-1 randomized controlled trial looked at both virologic and clinical outcomes in 452 outpatients after treatment with bamlanivimab, a SARS-CoV-2-spike protein-targeting monoclonal antibody (N Engl J Med 2021;384:229-37). Patients were randomized to one of four treatment arms: (700 mg, 2,800 mg, 7,000 mg, or placebo). Viral load at 11 days, measured by the cycle-threshold of the RT-PCR assay, was the primary endpoint. The study also looked at symptom score and emergency department visits or hospitalizations. To the authors' credit, only viral load, the primary outcome variable, was tested for statistical significance.

While the three antibody doses decreased viral load, only the 2,800-mg dose produced a statistically significant drop in cycle threshold (P=0.02). The 7,000-mg dose performed the worst (P=0.7). Only 1.6% of patients who received bamlanivimab required hospitalization, while 6.3% of those receiving placebo required hospitalization. Additionally, the clinical burden, assessed by a survey of multiple symptoms, resolved more quickly in those receiving bamlanivimab.

The interim analysis led to the EUA for bamlanivimab on November 9 last year, the first monoclonal therapy approved for COVID-19. The EUA authorized bamlanivimab for patients testing positive for SARS-CoV-2 who are 12 years or older and at risk for progressing to severe COVID-19 and/or hospitalization.

Bamlanivimab/etesevimab cocktail

The BLAZE-1 randomized controlled trial continued with bamlanivimab 2,800 mg combined with etesevimab 2,800 mg in an antibody “cocktail.” Etesevimab is another monoclonal antibody directed against the spike protein. As before, the primary endpoint was the viral load at day 11, with secondary endpoints including hospitalizations and clinical symptoms.

The combination antibody cocktail significantly decreased viral load on day 11 when compared with placebo (P = 0.01)(JAMA January 2021). Only 1% of patients with the monoclonal cocktail required hospitalization, compared with 6% of patients in the placebo group (P=0.049). Treatment with the monoclonal cocktail reduced symptoms from one week through 22 days after, compared with patients in the control group, but this only reached statistical significance (P<0.05) on day 11. The only adverse event in any patient was a urinary tract infection, which was considered unrelated to the study drug.

On February 9, 2021, the FDA issued an EUA for the co-administration of bamlanivimab and etesevimab in a monoclonal cocktail to treat mild-to-moderate COVID-19 in those age 12 or older who are at risk for progression to severe COVID-19. The EUA also allowed the drugs to be infused over just 20 minutes, significantly faster than the 60 minutes required by the initial EUA.


Regeneron, a competitor of Lilly, actively pursued a cocktail of two monoclonal antibodies, casirivimab and imdevimab, after seeing promising results in rhesus monkeys (Science 2020;370:1110-5). Like Lilly, they anticipated that a two-antibody cocktail would make it more difficult for resistant variants to emerge. The rationale is the same as our use of a three-drug regimen for tuberculosis to prevent emergence of resistant strains of TB. Their two antibodies were also directed at the spike protein. In a double-blinded RCT, Regeneron compared placebo, 2.4 g of REGN-COV2, and 8.0 g of REGN-COV2 (N Engl J Med 2021;384:238-51). Similar to the BLAZE-1 trial, their primary outcome variable was viral load (at seven days). The prespecified clinical endpoint was the percent of patients with at least one COVID-19 related medical visit through day 29.

As shown in Figure 1, both doses of REGN-COV2 decreased viral load compared with placebo. Six percent of patients in the placebo group required a medical visit, while only 3% in the combined treatment groups required a visit. There was no difference in the safety analysis between treatment and control patients.

Additional N=1 evidence was accumulated when President Trump was treated with REGN-COV2 at Walter Reed hospital following his diagnosis of COVID-19.

Based on the accumulated evidence, on November 21, the FDA issued an EUA for the monoclonal antibody cocktail of casirivimab and imdevimab (REGN-COV2, Regeneron) to treat mild-to-moderate COVID-19 in patients 12 years or older who were at risk for progressing to severe COVID-19.

Future directions

The two approved antibody cocktails, impressive as they are, are only the first steps in creating antibodies against SARS-CoV-2. Using an approach called “directed-evolution,” scientists engineered an antibody, ADG-2, that shows strong affinity to the receptor-binding domain of the spike protein (Science January 2021). ADG-2 is not specific to SARS-CoV-2, but is highly efficient against the all SARS betacoronaviruses. It also binds avidly to the N501Y mutation in the rapidly spreading B.1.1.7 variant.

The authors noted, “ADG-2 binds to a highly conserved motif {in the spike protein} through a distinct angle of approach.... This epitope may represent an ‘Achilles’ heel' for clade 1 sarbecoviruses.... ADG-2 represents a promising candidate for the prevention and treatment of not only COVID-19, but also future respiratory diseases caused by pre-emergent SARS-related CoVs.” If subsequent studies bear this out, ADG-2 could be stockpiled as an “off-the-shelf” monoclonal antibody the next time a betacoronavirus jumps into the human population.

“Single-domain antibodies” are super-compact antibodies ( They are typically a peptide chain of about 110 amino acids but have similar ability to bind antigens as conventional antibodies. Their small size allows them to reach antigens that conventional antibodies are too bulky to reach. We don't make these – they were first identified in camels. However, we can synthesize them in a lab.

There are many potential benefits to single domain antibodies: 1) they can bind to antigens (e.g., pieces of the spike protein) that conventional antibodies are too bulky to reach, 2) they are really cheap to make, 3) they are stable so special storage and handling isn't required, and 4) they can be absorbed through the lungs and administered with a simple nebulizer. Because the hard-to-get-to targets of single-domain antibodies aren't visible to our human immune system, there isn't strong selection pressure for mutations.

Scientists have created several single-domain antibodies that have very strong binding to highly conserved portions of the SARS-CoV-2 spike protein, with no diminution of activity against known variants (Science February 2021). The single domain antibodies are able to lock the spike protein in an “active” configuration, which results in the receptor binding domain cleaving from the protein as shown in Figure 2. The result is an inactive, noninfectious spike.

Quoting the authors: “Perhaps, in the future, a positive rapid SARS-CoV-2 test outcome will go hand in hand with an easily administered, affordable, subcutaneous injection or nebulized inhalation of an antibody targeting highly conserved epitopes not recognized by the human immune system.” In other words, as soon as you have a positive test, you are given a nebulized treatment rendering it impossible for the COVID-19 to progress.

Why are deaths so high?

Monoclonal antibodies have been available since late November. Since effective therapy has been available, why did COVID-19 deaths surge to new peaks in January? One reason, of course, is that cases surged because of SARS-CoV-2 seasonality as well as the superspreading events of the Thanksgiving and Christmas holidays. However, even with surging cases, shouldn't deaths have decreased dramatically with the availability of these cocktails?

This hasn't been well studied, but several explanations have been floated in the media as to why deaths are surging and antibody cocktails sit, unused, on pharmacy shelves (N Engl J Med 2021;384:289-91;

  1. Treatment for COVID-19 is advancing so fast that many clinicians are not aware that these are available.

  2. There is a mistaken belief that these are for inpatient use. That may arise, in part, because the approved cocktails must be given by I.V. infusion. However, the intent is for the drugs to be administered to outpatients early in the course of the disease.

  3. The reason to give the cocktails early in therapy is that they must be given before the immune system goes haywire. As COVID-19 progresses from moderate to severe disease, end-organ damage changes from viral-induced to autoimmune injury. At that point, decrease viral load is of limited benefit.

William Haseltine was right: monoclonal antibodies have been quickly developed. They have the potential to radically change therapy. As always, science has progressed faster than clinical medicine. Monoclonal antibody cocktails should be considered on the initial presentation of any patient with COVID-19 at risk for serious illness.