The SARS Cov-2 antibodies tests should not be used as a reliable marker for non replicating virus. It is not known at what pace and timeframe antibody levels decline over time, nor is it certain that the presence of antibody levels, or certain antibodies indicate effective immunity.
Many different antibody tests are available at this time. Those with FDA approval are summarized on the FDA Emergency Use Authorization webpage1.
Additional antibody tests are often available for research use within institutions. When considering use of serology testing, clinicians should look at its performance characteristics in terms of sensitivity, specificity, positive predictive value, and negative predictive value. Manufacturer provided sensitivity and specificity are not necessarily accurate; larger real-world studies are generally needed for most antibody tests. The tests are not interchangeable.
Many of the tests are directed at IgG, which develops in the later stages of infection. Others, however, may be directed at IgM and/or IgA, which develop earlier in the time course in most viral infections, may or may not be the case in SARS CoV 2 Furthermore, the target sequence of the antibody tests differ significantly.
Three types of tests are available: 1) Direct reading tests, 2) ELISA, and 3) Neutralizing antibodies. The direct reading tests can provide rapid responses but must generally be performed one test at a time and are therefore not scalable. ELISA testing can provide a quantitative titer, although results are sometimes reported only as positive or negative. Neutralizing antibody tests may be the most relevant to the question raised since they directly ascertain if the antibody can kill the virus. However, neutralizing antibody tests are much more difficult to perform and are therefore rarely available.
Currently, antibody tests have limited utility in managing individual cases. Rather, they are useful for epidemiologic analyses on a research basis or to describe factors associated with infection in a particular population group. They have potential for monitoring the overall effectiveness of a prevention program within an employer or other populational setting.
Neither the presence of antibody nor the quantitative level can reliably be used to determine if patients who have continued shedding of viral material as measured by PCR or antigen testing are actually shedding virus capable of replication. The role of cellular immunity in long term protection is not yet defined but may be important.
Despite these caveats, it is reasonable to anticipate that the presence of antibody, particularly late phase IgG, may be associated with a reduced likelihood of shedding active virus. It is also logical that persons with antibody are at much lower risk of reinfection than others. While logical, current data are insufficient to confirm that these hypotheses are valid.
Emerging data suggest asymptomatic individuals with SARS-CoV-2 infections had a weaker immune response to the viral infection. There has been some reduction in IgG and neutralizing antibody levels in the early convalescent phase which has implications in terms of sustained immunity against the infection2.
This area is rapidly evolving, and it is hoped that their utility may be much greater in the near future.