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Currently available antibody tests may not be reliable or validated. Even if these tests reliably identify antibodies to SARS-CoV2, this may not indicate current or future immunity to COVID-19. Professional and scientific organizations including the American Society for Microbiology, Infectious Diseases Society of America, American Clinical Laboratory Association, and the CDC as well as the World Health Organization have all issued concordant recommendations regarding the use of serology. Serologic tests may indicate prior infection and may be useful for epidemiologic research. However at this time serology cannot be used to determine protective immunity, and should not be used to make workplace decisions such as work assignment or PPE. Testing employees for antibodies is not recommended until many key issues are addressed:
- Antibody tests ascertain the presence of IgM, IgG, and IgA. IgG is the antibody that could indicate immunity, although future studies may evaluate the role of IgA.
- Two approaches are most commonly used to test for anti-SARS-CoV-2 antibodies: lateral flow assays (LFAs) and ELISAs. LFAs are qualitative tests that have bands that need to be read by a trained reader. ELISA tests are quantitative and may be more accurate but are more difficult to perform.
- Tests are not reliable. On May 4, FDA ordered companies to submit antibody test data for marketed tests in the next 10 days after scientists demonstrated that many commercially available tests are not reliable.1,2 This study looked at 14 tests, but more than 100 tests are currently sold. The authors state: “High rates of positive results were not reached until at least 2 weeks into clinical illness; diagnosis at time of symptom onset thus remains dependent on viral detection methods.”
- There needs to be proof that tests are not demonstrating antibodies to the four coronaviruses that cause “common cold” symptoms — 229E, OC43, NL63 and HKU1 — which are highly prevalent. Tests need to distinguish between these viruses and SARS-CoV-2. Some tests react with pre-COVID-19 plasma (108 blood donor plasma specimens collected before July 2018), suggesting that they are reacting to another coronavirus or something in the media.2 Only one of the 14 assays had no false positives.
- Since COVID-19 is still uncommon, low population prevalence means low pre-test probability, which increases the risk that positive results are false positives. A false positive result would suggest that an individual could have immunity, when they do not. It is important to know the specificity and sensitivity of any test, so that the false negative rate can also be calculated.
- Further study is needed to show whether antibodies produced against SARS-CoV-2 are neutralizing antibodies that could produce immunity. Per WHO, “At this point in the pandemic, there is not enough evidence about the effectiveness of antibody-mediated immunity to guarantee the accuracy of an ‘immunity passport’ or ‘risk-free certificate’.”3
- Mutations that are occurring in SARS-CoV-2 could lead to strains that are not recognized by antibodies to other strains. Immunity to one strain would not guarantee immunity to another.4
Estimates based on other coronaviruses suggest that if immunity occurs, it may only last from a few months to 2-3 years.5-7 This could result in someone having a
positive test for antibodies while they are no longer immune.