Laboratory testing has played a key role during the COVID-19 pandemic with molecular and antigen tests being used for screening and diagnostic purposes. However, SARS-CoV-2 serological testing was less utilized due to various limitations of the test. Analytical limitations include poor test sensitivity early in infection, lack of standardization, and differential detection of antibodies produced by viral variants. Arguably, test sensitivity was more problematic in the initial stages of the pandemic when there was historically low COVID-19 seroprevalence. Given significantly higher infection and vaccination rates in the current population, lower sensitivity tests can be informative since prevalence of antibody levels are higher (Brownstein et al. Sci Rep 2021). Lack of standardization however remains problematic despite the development of a WHO standard (Kristiansen et al. Lancet, 2021). Commercial vendors continue to report in different measuring units and across different analytical measuring ranges. To further complicate matters, the antigen targets (nucleocapsid, spike or receptor binding domain) and antibody isotypes (IgG, IgA, IgM, total antibody levels) differ between instrument platforms. Finally, viral variants are another contributor to analytical challenges. Conformational changes to SARS-CoV-2 viral particles can alter the binding affinity and shape of the associated antibodies, which may in turn reduce the ability of the assay to detect such antibodies (SARS-CoV-2 Viral Mutations).

There are also several physiological factors that impede test interpretation. These include: overlaying infection and vaccination status, lack of immune response in specific populations, large in-between individual biological variation, and unknown titers conferring immune protection (Fung et al. CRCLS 2023). Antibody kinetics have been extensively studied and have shown a variable response between individuals (Garcia et al. Viruses 2022). Observations include differences in seroconversion rate, antibody titers, neutralization capability, and sero-reversion rate. Multiple patient characteristics have also been identified to play a role in the variable response observed including symptom severity, age, body mass index, sex, and comorbidities (ie/ immunosuppression, diabetes, etc) (Fung et al. CRCLS 2023). Perhaps the largest challenge is the unknown antibody level that is associated with immune protection. As serological testing for other infection diseases is used clinically in this way, for example hepatitis, the complex relationship between immune protection and SARS-CoV-2 antibody levels has not been clearly defined, limiting the test’s clinical utility.

Due to the above limitations, SARS-CoV-2 serological testing played a minimal role in the clinical setting. However, it was heavily used in research to facilitate critical decisions throughout the pandemic. A primary example is the assessment of SARS-CoV-2 neutralizing antibody levels in clinical trials of COVID-19 vaccines, which was linked to vaccine effectiveness (Yang et al. The Lancet 2023). In a similar way, COVID-19 antibody levels were assessed to make decisions regarding length of time between vaccine doses, and identified high risk individuals who required more frequent vaccination. Through sero-surveillance studies, serological test results highlighted the prevalence in asymptomatic populations and provided guidance for public health measures aimed at reducing the spread of COVID-19 (Bobrovitz et al PLoS ONE 2021). Moreover, by assessing antibody isotypes, further information was derived regarding recent vs previous infections and neutralization capacity in these patient populations (Noval et al. Scientific Reports, 2021).

Where are we now? There is a possibility that SARS-CoV-2 serological testing may be used for immune assessment in the future, but current practices have not integrated serological testing into routine clinical care. Nonetheless, SARS-CoV-2 serology testing remains useful as research continues to unfold. Measuring serological response alongside T-cell response using novel biosensor platforms may also be informative as T-cell memory responses have been shown to correlate with COVID-19 disease severity (Racine-Brzostek et al. Clin Chem 2021, Zhao et al. Viruses 2023). Another on-going area of investigation is understanding the serological response in patients with long COVID and assessing future vaccine strategies. As society returns to a new normal, SARS-CoV-2 related laboratory testing have fallen to a lower volume, but the future always remains unpredictable.

References

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