Convalescent Plasma and Passive Antibody Therapy

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No vaccine has yet been created for COVID-19, but one treatment has been gaining more and more attention in the fight against this disease: passive antibody therapy using convalescent plasma. Before we can discuss how antibody therapy works, two concepts must be defined: plasma and antibodies.

Plasma is the liquid component of the blood. While many people think of blood as a red substance, only 45 percent of blood contributes to the red color. This is the solid part of the blood, composed of red blood cells to carry oxygen, white blood cells to fight infection, and platelets to help with clotting. The liquid plasma that makes up around 55 percent of the blood is actually yellow in color. Plasma is made of water, salts, and proteins. The purpose of plasma is to transport nutrients, hormones, proteins to their appropriate space in the body, as well as to get rid of waste.

 

Photo: Components of blood

Antibodies are the body’s soldiers to fight against infections. When the body is invaded by a foreign substance, or pathogen, the immune system will work to rid the body of this invader. Examples of pathogens include bacteria, viruses, and other organisms that can cause disease. Pathogens contain small molecules called antigens that the body recognizes as “foreign”. Antibodies are proteins produced in response to these antigens. Once these antibodies are produced, they will bind to the antigen and inactivate it. Antibodies have specificity, meaning they can only bind to a specific antigen. For example, SARS-CoV-2 antibodies are only effective against SARS-CoV-2, not against any other viruses.

Individuals who have recovered from a SARS-CoV-2 infection will have SARS-CoV-2 antibodies in their blood plasma. This is called active immunity because the individual’s immune system has created its own antibodies. Passive antibody therapy is based on the idea that these antibodies can be transferred to an infected person to help them fight the disease – it is passive because the immunity is received from another person’s antibodies. Potentially, these antibodies could also be used as a preventative measure, or prophylaxis. In this method, antibodies would be given to individuals who are at high risk of contracting the disease in order to prevent infection.

How can these antibodies be given to another person? By transferring the blood plasma. Convalescent plasma is plasma that has been collected from a person who has recovered from a disease – this plasma will contain the antibodies for that particular disease. Just like a regular blood transfusion, plasma must match the recipients blood type. However, one person’s plasma can be used to treat multiple people.

Passive antibody therapy using convalescent plasma is not a new concept. Plasma has been used to treat diseases since the early 1900’s; however, it was largely abandoned in the 1940’s in favor of antibiotics. Recently, antibody therapy has seen a resurgence.

One recent event that saw the use of antibody therapy was the SARS-CoV outbreak. During this epidemic, a hospital in Hong Kong treated 80 patients by using convalescent plasma. The mortality rate for patients treated with plasma was lower than the typical mortality rate of SARS-CoV, showing that the convalescent plasma treatment improved the odds of a positive outcome in patients. Additionally, more positive outcomes were seen in patients who were given the plasma earlier in the course of their disease. This shows that convalescent plasma is more effective when it is used as a treatment shortly after infection rather than as a last resort.

Currently, hospitals around the world are using convalescent plasma to help treat infected patients. Convalescent plasma clinical trials are ongoing in China, Colombia, France, the Netherlands, and Spain. While the effectiveness of antibody therapy for SARS-CoV-2 is still largely unknown and needs more research, small studies in China and Korea showed promising results. Both studies concluded that antibody therapy was a specific and effective method of treatment. However, both studies treated COVID-19 patients who had already been infected by COVID-19, so the prophylactic efficacy of this treatment is unknown.

There are certain risks that come with using convalescent plasma. Since plasma is a part of the blood, there is a risk of transferring blood borne pathogens to the recipient. However, blood screening is very effective, so this risk is low. Another potential risk is that convalescent plasma will trigger antibody-dependent enhancement of infection (ADE). Some coronaviruses have an ADE mechanism in which certain antibodies can increase the severity of the disease. It is currently unknown whether SARS-CoV-2 antibodies will result in ADE. Despite these possible risks, antibody therapy could prove to be an effective weapon in the fight against COVID-19.

References:

Cormac Sheridan. “Convalescent serum lines up as first-choice treatment for coronavirus.” Nature Biotechnology (May 2020).

Giuseppe Marano. “Convalescent plasma: new evidence for an old therapeutic tool?” Blood Transfusion 14(2): 152–157 (Mar 2016).

Kai Duan. “Effectiveness of convalescent plasma therapy in severe COVID-19 patients.” Proceedings of the National Academy of Sciences of the United States of America 117(17) (Apr 2020).

Mingxiang Ye. “Treatment With Convalescent Plasma for COVID-19 Patients in Wuhan, China.” Journal of Medical Virology (Apr 2020).

Y. Cheng. “Use of convalescent plasma therapy in SARS patients in Hong Kong.” European Journal of Clinical Microbiology and Infectious Diseases 24: 44–46 (2005).

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