Measuring immunity to SARS-CoV-2 infection: comparing assays and animal models

Abstract

The rapid scale-up of research on coronavirus disease 2019 (COVID-19) has spawned a large number of potential vaccines and immunotherapies, accompanied by a commensurately large number of in vitro assays and in vivo models to measure their effectiveness. These assays broadly have the same end-goal — to predict the clinical efficacy of prophylactic and therapeutic interventions in humans. However, the apparent potency of different interventions can vary considerably between assays and animal models, leading to very different predictions of clinical efficacy. Complete harmonization of experimental methods may be intractable at the current pace of research. However, here we analyse a selection of existing assays for measuring antibody-mediated virus neutralization and animal models of infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and provide a framework for comparing results between studies and reconciling observed differences in the effects of interventions. Finally, we propose how we might optimize these assays for better comparison of results from in vitro and animal studies to accelerate progress.

SARS-CoV-2 antibody testing for estimating COVID-19 prevalence in the population

“…Extensive washing between these detection steps reduces background signals, making ELISAs highly accurate. 46 CLIAs are similar to ELISAs but rely on coated microparticles than immobilized antigens, also yielding high accuracy with excellent amenability for automation.…”

“…Downsides of this approach are rather low throughput, a requirement for high safety precautions (because the experiments require working with infectious SARS-CoV-2), and a rather long incubation period until results are available (it takes several days for lysis of cells to become visible as plaques). 46 Several variations and alternatives exist for the plaque-reduction neutralization assay (PRNTs) recently reviewed by Khoury et al. 49 These include replacing live SARS-CoV-2 with replication-defective pseudoviruses in which the SARS-CoV-2 S protein is incorporated into the surface of vesicular stomatitis virus or lentiviruses (single-cycle virus neutralization assays).…”

“…Positive and negative predictive value (PPV and NPV, respectively) are metrics representing test performance at a certain prevalence in the population (reviewed in detail by Gronvall et al. 46 ).

Figure 1 The effect of sensitivity and specificity on antibody testing results is linked to the true prevalence in the population (A) Schematic overview of the effect of test sensitivity/specificity at different prevalence in the population, illustrating false positives and false negatives.

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“… 82 , 83 For example, highly infectious measles requires more than 90% immune individuals to achieve herd immunity. 49 , 92 For SARS-CoV-2, the exact threshold to achieve herd immunity is unclear, 85 and estimates are complicated by differences between countries, with most estimations between 50% and 80%. 86 Currently reported positive rates in antibody testing ( Table 2 ) are well below estimated thresholds required to achieve herd immunity, even in the epicenters of the outbreak.…”

“…Furthermore, there are several caveats requiring careful consideration when leveraging antibody testing results for assessing herd immunity. 46 , 56 A positive result of an antibody test (even leaving testing inaccuracies aside), does not necessarily indicate protection against reinfection of an individual. The produced antibodies may not neutralize the virus, and the necessary magnitude of the immune response is also unclear (i.e., very low antibody concentrations detected by sensitive tests may not be sufficient for viral clearance of a reinfection).…”