A key part of the immune response in humans is for specific white cells in the body (called B cells) to produce a protein that can attach (bind) to the virus and disable it. These proteins are called antibodies. After overcoming an illness like COVID-19, people have circulating levels of antibodies that can disable or neutralize the virus to protect them from repeat infections. The body can retain the ability to produce these antibodies for years or decades in some cases. An alternative way to protect someone is therefore to give them an infusion of antibodies that are targeted against the virus. When your body makes antibodies against a virus there are usually many different kinds that bind different parts of the virus, this is called a polyclonal antibody response. If one of these single types of antibodies—an antibody that is particularly good at binding to and neutralizing the virus—is cloned out so that many antibodies can be made that are all the same, these are called monoclonal antibodies. Protein drugs made from a single clone of antibody or monoclonal antibody (“mAb”) have become a mainstay of medical therapy since the approval of the first therapeutic mAb in 1986. More recently there has been a focus on using mAbs to fight infectious diseases with the idea that mAbs can bind to and neutralize a range of pathogens. There have been many studies also looking at using mAbs as a preventative treatment against infectious diseases including cytomegalovirus, Ebola, influenza, HIV, rabies and respiratory synctitial virus (RSV). One mAb, Palivizumab (Synagis, MedImmune), is already approved by the FDA to protect infants from RSV.
One key advantage of mAbs for infectious diseases is that they can be given regardless of the patient’s infection status. Even if intended to prevent disease, they can help neutralize virus in an active infection. While they may not be curative in all people, they can offer more help to the infected person’s immune system than a vaccine.
Artistic rendering of monoclonal antibodies (in red) binding to a virus. Source: Science Clarified
Monoclonal antibodies are typically used to treat disease. But here we are interested in preventing disease. The number of sick people to be treated for a disease may be limited, but the number of people that need to be treated to prevent a disease is much, much larger. For the current coronavirus infection, the number of people that will need to be vaccinated or treated will be in the millions. In the face of this required scale, mAbs have two important limitations. The first is cost. Therapeutic antibodies can be priced at tens and hundreds of thousands of dollars. This is obviously not a practical approach to protecting millions of people from infection. Part of the challenge in the use of antibodies is their relatively high cost of production. Facilities to produce mAbs for treating large populations can cost $300-$500 million to build and require thousands of people to staff. They are also expensive to produce, with costs of commercial mAbs at about greater than $100 per dose. The other limitation of antibodies is that many of them require intravenous administration, which again is not feasible when millions of people need to be treated or protected.