What is antibody testing?
An antibody test, also called a serology test, measures the number of specific antibodies present in the blood when the body is responding to a particular infection such as COVID-19 caused by the virus, SARS-CoV-2. In response, the body develops antibodies that attach to the virus to help eliminate it. The body’s immune reaction initially produces antibodies called “IgM”, which indicate an active or recent infection. If IgM antibodies are absent, it does not mean that a particular individual is not infected since it takes time for the body to make these antibodies. Furthermore, a second type of antibody develops over time called “IgG” that takes about four weeks on average to develop. However, the time to develop IgG antibodies can vary substantially1.
What is interesting is what these antibody tests will mean. For instance, some pathogens such as the varicella- zoster virus, the causative agent for chicken pox, can develop long-lasting immunity. However, the human immunodeficiency virus (HIV) often produces a large amount of antibodies that do not prevent or clear the virus. Furthermore, immunity to other seasonal coronaviruses starts to decline a couple of weeks after infection. Within a year, some people are still vulnerable to reinfection. Studies of SARS-CoV, the virus that causes severe acute respiratory syndrome (SARS) and shares a large amount of genetic material with SARS-CoV-2, shows immunity peaking around four months along with protection for roughly two to three years2.
The U.S. Food and Drug Administration (FDA) understands the public health value in expanding the availability of testing for SARS-CoV-2 through safe and accurate tests that may include home collection1. Additionally, several surveys to test for antibodies have been launched around the world. The World Health Organization (WHO) announced a global effort called Solidarity II, which will be a coordinated study to test blood samples for the presence of antibodies to SARS-CoV-2 involving more than half of a dozen countries. In the United States, a collaborative project launched with the aim to provide a “picture” of nationwide antibody prevalence. During its first phase, samples are already being collected from blood donors in six major urban areas, which include New York City, Los Angeles, Minneapolis, Boston, San Francisco, and Seattle. The Centers for Disease Control and Prevention (CDC) will then generate three national surveys of donors to be conducted in September and December of 2020, and November of 2021. This will help to determine if antibody responses are waning and to assess for herd immunity2.
What is herd immunity?
According to the CDC, herd immunity is protection from a disease in a group due to a large enough proportion of the population having immunity to prevent the disease from spreading from person to person3. For example, if 80 percent of a population has immunity towards a virus then four out of every five people who encounter someone with the virus will not get sick or spread the virus. Depending on how contagious an infection is, usually 70 percent to 90 percent of a population needs immunity in order to achieve herd immunity4.
What are some concerns with antibody testing?
Unfortunately, according to the WHO, it is unclear whether the presence of these antibodies in the blood will give full protection against reinfection with SARS-CoV-25. The current antibody testing landscape remains clinically unverified. These tests should also not be used for sole diagnosis. There are more considerations including the potential for cross-reactivity with other coronavirus antibodies, variable test detection limits, and the need for validation from clinical laboratories6.
It is still unclear whether the presence of antibodies will warrant protection against SARS-CoV-2. Of course, information is rapidly evolving during this difficult time. Hopefully, there will be more research, laboratory funding, and public health guidance that will become available in order to help direct testing and the appropriate response towards SARS-CoV-2.