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Background The association between SARS-CoV-2 commercial serological assays and virus neutralization test (VNT) has been poorly explored in mild patients with COVID-19. Methods 439 serum specimens were longitudinally collected from 76 healthcare workers with RT-PCR-confirmed COVID-19. The clinical sensitivity (determined weekly) of nine commercial serological assays were evaluated. Clinical specificity was assessed using 69 pre-pandemic sera. Correlation, agreement and concordance with the VNT were also assessed on a subset of 170 samples. Area under the ROC curve (AUC) was estimated at 2 neutralizing antibody titers. Results The Wantai Total Ab assay targeting the receptor binding domain (RBD) within the S protein presented the best sensitivity at different times during the course of disease. The clinical specificity was greater than 95% for all tests except for the Euroimmun IgA assay. The overall agreement with the presence of neutralizing antibodies ranged from 62.2% (95%CI; 56.0-68.1) for bioMérieux IgM to 91.2% (87.0-94.2) for Siemens. The lowest negative percent agreement (NPA) was found with the Wantai Total Ab assay (NPA 33% (21.1-48.3)). The NPA for other total Ab or IgG assays targeting the S or the RBD was 80.7% (66.7-89.7), 90.3 (78.1-96.1) and 96.8% (86.8-99.3) for Siemens, bioMérieux IgG and DiaSorin, respectively. None of commercial assays have sufficient performance to detect a neutralizing titer of 80 (AUC<0.76). Conclusions Although some assays show a better agreement with VNT than others, the present findings emphasize that commercialized serological tests including those targeting the RBD cannot substitute a VNT for the assessment of functional antibody response.

Following severe adverse reactions in patients vaccinated with the AstraZeneca ChadOx1 (Chad) vaccine, European health authorities have recommended that patients under the age of 55 who received one dose of Chad vaccine receive a second dose of Pfizer BNT162b2 (BNT) vaccine as a booster. However, the effectiveness and the immunogenicity of this vaccination regimen have not been formally tested. Here, we show that the heterologous Chad/BNT combination confers better protection against SARS-CoV-2 infection than the homologous BNT/BNT combination in a population of health care workers. To understand the underlying mechanism, we monitored in a longitudinal way the anti-spike immunity conferred by each vaccinal combination. Both combinations induced strong anti-spike antibody responses after boost in all vaccinated individuals. However, sera from heterologous vaccinated individuals displayed a stronger neutralizing activity, regardless of the SARS-CoV-2 variant analyzed, and this was associated with more switched memory RBD-specific B cells with an activated phenotype and less IgA. The Chad vaccine induced a stronger T cell response than the BNT vaccine after the priming dose, and the reciprocal was true for the IgG response, which could explain the complementarity of both vaccines when used in an heterologous setting. This strongly protective vaccination regimen could be therefore particularly suitable for immunocompromised individuals.