Knowing the Score: Antibody Testing and Immunity

Last updated on May 14, 2020

         A few weeks ago, we talked about antibodies and how they can convey immunity after an infection or a vaccine. Recently, there has been a lot of talk about antibody tests and whether they could be used to allow “immune” individuals to return to work. But there are a lot of misconceptions about these tests, what they actually tell us, and how they should be implemented. There is a persistent misconception that if an individual has antibodies, they are completely immune to reinfection. It is completely possible, even likely, that if you got the coronavirus and barely had any symptoms, then you may not have enough antibodies to prevent reinfection. Even as some tests can calculate an exact number of antibodies a person has, doctors don’t yet have an understanding of just how many antibodies are required to convey immunity. And no one is sure how long that immunity will last.

         Antibody tests (also known as serological tests) identify the presence of antibodies in the blood, which may be indicative of a past infection. This can be helpful for epidemiologists trying to determine the course of a disease (especially in the case of Covid-19 where the majority of infected may have had little to no symptoms). Additionally, blood from people identified to have antibodies (convalescent plasma) may be a helpful treatment for severe cases of Covid-19. A transfusion of convalescent plasma could help a struggling patient’s immune system identify and eliminate the threat faster.   

         In some cases, antibody tests are positive indicators of immunity. Both measles and chickenpox create strong lifelong immunity through antibodies and memory B-cells that remember the virus and prevent reinfection. Other infections provide more temporary immunity, weaker immunity, or even no immunity at all. HIV is an extreme example of a virus that doesn’t convey any immunity—infected individuals produce a large amount of HIV antibodies, but these antibodies are completely ineffective at neutralizing a secondary infection.

         But what about SARS-CoV-2? How effective are Covid-19 antibodies at providing immunity? The answer is that we don’t really know yet. Preliminary studies have indicated that individuals infected with SARS-CoV-2 develop some level of neutralizing antibodies (the specific antibodies that confer protective immunity). But it is unknown how long this immunity will last. It is also unclear how many useful antibodies the initial infection generates. The majority of people infected with Covid-19 have mild symptoms or even no symptoms at all, which could mean fewer antibodies created. Initial reports indicate that individuals with mild symptoms may still generate enough antibodies for immunity, but more studies are needed to determine the strength and length of immunity. 

         The first coronavirus antibody test was given emergency approval by the FDA on April 1^st. Since then, elevenmore antibody tests have been given emergency approval. The quality and reliability of these tests can vary—as can the usefulness of their results. There are three main types of antibody tests: rapid diagnostic tests (RDT), enzyme-linked immunosorbent assays (ELISAs), and neutralization assays.

         RDTs are the fastest form of antibody test, taking around 10-30 minutes to yield results. Blood is dropped on a test strip and moves up the strip, encountering specific Covid-19 antigens (protein on the outside of virus that antibodies recognize), as well as tagged control antibodies. Further up the strip, a test line containing the antibody of interest catches antigens that have been pulled up with the antibodies in the blood. The blood continues even further up the strip where a control line catches the tagged control antibodies verifying that the test worked normally. If both the control line and the test line show color, then the test is considered positive for Covid-19 antibodies. If only the control line shows color, then the test is negative for antibodies. Any result where the control line shows no color is considered inconclusive.

         RDTs can only verify whether a person has antibodies or not. These tests don’t give any quantitative results concerning the number of antibodies in the blood, and they don’t give any indication as to whether those antibodies are able to neutralize a Covid-19 infection. However, false positives can be a big issue with RDTs. While most of these tests have about a 5% chance of providing false positives, this error inflates when the number of infected individuals in a population is small. 

         Consider a small-town of 1000 people. 150 people in the town have been exposed to Covid-19. Everyone in the town takes the antibody test to figure out if they are immune. Our test correctly gives positive results to 95% of people with antibodies and correctly gives negative results to 95% of people with negative antibodies. So, of the 150 people with antibodies, 7.5 people (7 or 8 people since you can’t have ¾^ths of a human) get a negative result. That’s not so bad, and maybe the worst thing that happens is that a few people who are “immune” can’t get a haircut. On the other end of the spectrum, of the 850 people who do not have antibodies, 42.5 people (42 or 43) get a positive result. A total of 185 people in the town are told that they can go back to work, get haircuts, and attend gatherings, but 23% of those people are actually vulnerable to reinfection. These tests may be accurate enough in places like New York, where the percentage of infected citizens is high enough to limit the effect of false positives, but in other areas of the country they may be far too unreliable.   

         Alternatively, ELISA tests take 2-5 hours to perform and are performed by trained lab technicians. A patient’s blood is incubated in tray wells containing the specific Covid-19 antigen. Another tagged antibody is deposited into the well where it binds to any antibody-antigen complexes in the blood. The degree of color change can be measured precisely with a device called a spectrophotometer. The specific amount of color change is linked directly to the concentration of antibodies in the blood sample.

         ELISA tests give more information about the number of antibodies a person has. If this was a disease that we had a lot of experience with, doctors would have a known immunity threshold (the number of antibodies needed to make a patient immune to reinfection). There is no known immunity threshold for Covid-19 yet, so the results from ELISA tests don’t actually give us enough information to establish immunity. It’s a bit like knowing the points your favorite football team scored at the Superbowl, but not knowing how many the other team scored. You still have no idea if your team won or lost. 

         Neutralization assays are the gold standard for verifying immunity, but they take up to five days to perform and require extensive training and safety measures. In this test, dilutions of the patient’s serum are mixed with live SARS-CoV-2 virus. The samples are spread on a plate containing host cells and monitored. Uninhibited virus activity will cause several plaques (areas with no cell growth). The presence of antibodies at different concentrations inhibits the formation of plaques by preventing the virus from replicating.

         Neutralization assays give us definitive reassurance that the antibodies a patient has actually inhibit viral replication—a key function of immunity. But because they utilize live virus particles, they require labs and technicians with high levels of biological safety certification. That, combined with the time each test takes, mean these tests are not viable for efficient wide-spread use. However, they may be ideal for understanding how a person’s immunity to Covid-19 changes over time.

         I hope this answered a lot of your questions about antibody tests. These tests are fundamental to our understanding of where and how Covid-19 has spread, but they have limited capability on an individual level. Feel free to get an antibody test, especially in the context of a study trying to get information about Covid-19. Just don’t expect a positive result to mean that you are completely immune. Continue to practice social distancing and good hygiene as much as you can.

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