Notes from the Doc

Expert Voices: Antibody Testing for COVID-19 Immune Response, But Not Immunity

Today’s Expert Voice is Leslie Hoglund, PhD, MEd, Clinical Assistant Professor of Public Health, School of Community and Environmental Health, Old Dominion University

As testing to confirm SARS-CoV-2 infection expands, another test is surfacing in the fight to mitigate and suppress the disease, COVID-19. Serology testing for antibodies is a common way to determine if a person had a prior infection and to understand how widespread a disease has transmitted within a population. Antibodies are molecules that your immune system creates as a defense against a foreign infectious protein to prevent it from infecting more cells. Antibody testing is not used to diagnose or replace testing for the virus itself. Human coronaviruses are familiar as there are seven known strains – four are associated with the common cold, two strains are associated with prior epidemics, and the novel strain that causes COVID-19.

Serological tests have rapidly come into production and use in the U.S. in the past month, and more to come. Of concern, it is hard to distinguish if antibodies are from COVID-19 or the other four that are the common cold. This is called cross-reactivity and would result in a false positive. The tests are designed to detect antibodies against the virus in serum, plasma, or whole blood specimens, though a simply finger-prick of blood is all that is needed. Antibody tests measure two types of antibodies, IgM and IgG. IgM antibodies indicate a patient has COVID-19 and IgG antibodies demonstrate recovery. Evidence of antibodies is not always detected in everyone who is infected or recovered, however. Their blood could then be donated as convalescent plasma is a possible treatment for those who are seriously ill from the disease. National blood bank supplies are very low (consider giving blood to help) so the use of convalescent plasma to treat severe cases is ideal.

There are many advantages to antibody testing, especially to learn the prevalence for surveillance purposes. The Food and Drug Administration (FDA) permits antibody testing under an Emergency Use Authorization (EUA) authority to strengthen public health’s response to national emergencies. Authorization of seven tests by the FDA is allowed under specific and strict guidelines and laboratory settings. Companies who develop an COVID-19 antibody test must work to validate controls for cross-reactivity, class specificity, and clinical agreement. The FDA is monitoring for fraudulent operators and poor-quality tests being sold to hospitals and other health care facilities. It is important to know there are many limitations and potential errors to conducting antibody testing.

Antibody tests are about understanding exposure to the virus and the subsequent immune response. An antibody testing study, conducted by the University of Southern California and the Los Angeles County Department of Public Health in April 2020, demonstrated that COVID-19 was far more prevalent than the number of confirmed cases reported by the county. The preliminary findings denote that many people were infected and transmitted it to others without knowing; this can help public health to better understand the asymptomatic spread of the virus. The National Institutes of Health (NIH) launched a national study to quantify undetected cases through antibody testing (Contact: to enroll); blood samples from upwards of 10,000 people will be collected to identify the enormity of the pandemic in the U.S. Antibody testing conducted during the pandemic offers a solid signal if public health measures are working, i.e., school  and non-essential workplace closures, physical distancing, mask wearing in public, stay at home mandates, quarantining after exposure, isolating if infected, etc.

Recent studies and a scientific brief published by the World Health Organization have corroborated the fact that persons who have recovered from COVID-19 and have antibodies are not protected from a second infection; it is likely a person can get the virus again. Presence of antibodies in an immune response is not sufficient and does not mean lifetime immunity. We do not yet know how long antibodies will remain in the body after a COVID-19 infection. Still, antibody testing is very important and critical for epidemiological understanding and use of serological data in the design of vaccines against COVID-19.

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