Antibody Tests & COVID-19 "Immunity": A False Sense of Hope
Updated: Aug 13, 2020
While the idea of giving COVID-19 immunity “passports” to people who have antibodies for the virus sounds like a promising step in the right direction, the tests that would supposedly guarantee them are far from perfect. COVID-19 antibody tests, otherwise known as serology tests, are blood tests that identify individuals who have been previously infected with the SARS-Cov2 virus by detecting antibodies for it. Antibodies are proteins created by the immune system in response to a virus to help fight it off and prevent future infections of the same virus. The accuracy of these tests is determined by certain epidemiological measures, one being sensitivity, the proportion of people who once had the disease and actually received a positive antibody test result, and the other being specificity, the proportion of people who didn’t have the disease and actually received a negative test result. While some popular tests such as Roche and Abbott boast sensitivities as high as 100%, there are a few key downfalls that make these measures much less accurate than they seem:
Many tests aren’t able to distinguish between antibodies for SARS-Cov2 and other coronaviruses: There is a known set of six human coronaviruses, some of which cause infections as mild as the common cold. Some serology tests are producing falsely positive results because they are identifying antibodies of coronaviruses other than SARS-Cov2.
Some people with mild or asymptomatic cases of COVID-19 produce undetectable amounts of antibodies: The majority of cases of COVID-19 are either asymptomatic or mild, and it has been found that a smaller amount of antibodies are produced in individuals that fall under this category. In a study published in Nature Magazine, “the virus-specific IgG levels in the asymptomatic group” were significantly lower relative to the symptomatic group, with IgG (immunoglobulin G) being one type of antibody. Tests are producing falsely negative results because there are not enough antibodies to be detected.
Initial antibody levels are reduced as time goes on: For most mild and asymptomatic cases of COVID-19, antibody levels quickly decrease within a few months of SARS-Cov2 exposure. In one study, 93.3% of asymptomatic individuals had a reduction in IgG levels. The strength of the immune response seems to correlate with the severity of the symptoms that each individual experiences. This means that an individual who has had COVID-19 may receive a negative antibody test result because the antibodies they originally had greatly decreased.
Individuals presently infected with the virus can still receive negative test results: It takes 1 to 3 weeks for the body to develop an antibody response to SARS-Cov2. If an individual with COVID-19 is tested in the period before the antibodies develop, then they will receive a false negative result and therefore may believe they do not have the virus.
Antibody tests are much more accurate in populations where there is a high prevalence of COVID-19 infections: In addition to sensitivity and specificity, positive predictive value (the probability that people who have a positive test result truly have antibodies) and negative predictive value (the probability that people who have a negative test result truly don’t have antibodies) are also important in determining the accuracy of a test. Both measures take into account how common SARS-Cov2 antibodies are in the population at the time (prevalence). To illustrate why higher prevalence makes for a more accurate test, the following example can be used: if you are running a test on 100 people and 5 falsely positive results occur, then you can claim the test is 95% specific. Now take the same population and say it has a prevalence of COVID-19 of 5%, then that means that only 5 people actually have the virus, and therefore there are 5 true positive results. Since there are 5 true positives (5% prevalence) and 5 false positives (95% specificity), then the test is still wrong half the time, making it very inaccurate and unreliable.
A positive antibody test result doesn’t necessarily mean immunity from re-infection: in South Korea, 160 people tested positive again after they have “recovered” from COVID-19. It is currently believed that rather than becoming infected again, the virus reactivated in these individuals after a period of dormancy. More research needs to be done to understand exactly how the virus behaves over a long period of time.
Given all of the downfalls of the current COVID-19 antibody tests, it is very risky to rely on their results and allow those who have received a positive antibody test result to resume their normal pre-pandemic lives. Instead, it is critical that everyone continues to follow proper social distancing and mask guidelines in order to protect themselves and others.