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ABSTRACT Serological tests are beneficial for recognizing the immune response against severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). To identify protective immunity, optimization of the chemiluminescent reduction neutralizing test (CRNT) is critical. Whether commercial antibody tests have comparable accuracy is unknown. Serum samples were obtained from COVID-19 patients (n = 74), SARS-CoV-2 PCR-negative (n = 179), and suspected healthy individuals (n = 229) before SARS-CoV-2 variants had been detected locally. The convalescent phase was defined as the period after day 10 from disease onset or the episode of close contact. The CRNT using pseudotyped viruses displaying the wild-type (WT) spike protein and a commercial anti-receptor-binding domain (RBD) antibody test were assayed. Serology for the B.1.1.7 and B.1.351 variants was also assayed. Both tests concurred for symptomatic COVID-19 patients in the convalescent phase. They clearly differentiated between patients and suspected healthy individuals (sensitivity: 95.8% and 100%, respectively; specificity: 99.1% and 100%, respectively). Anti-RBD antibody test results correlated with neutralizing titers (r = 0.31, 95% confidence interval [CI] 0.22–0.38). Compared with the WT, lower CRNT values were observed for the variants. Of the samples with ≥100 U/mL by the anti-RBD antibody test, 77.8% and 88.9% showed ≥50% neutralization against the B.1.1.7 and the B.1.351 variants, respectively. Exceeding 100 U/mL in the anti-RBD antibody test was associated with neutralization of variants (P < 0.01). The CRNT and commercial anti-RBD antibody test effectively classified convalescent COVID-19 patients. Strong positive results with the anti-RBD antibody test can reflect neutralizing activity against emerging variants. IMPORTANCE This study provides a diagnostic evidence of test validity, which can lead to vaccine efficacy and proof of recovery after COVID-19. It is not easy to know neutralization against SARS-CoV-2 in the clinical laboratory because of technical and biohazard issues. The correlation of the quantitative anti-receptor-binding domain antibody test, which is widely available, with neutralizing test indicates that we can know indirectly the state of acquisition of functional immunity against wild and variant-type viruses in the clinical laboratory.

ABSTRACT Vaccines against severe acute respiratory syndrome coronavirus-2 have been introduced. To investigate the relationship between vaccine-induced humoral immunity and patient age, we measured antibody levels and neutralization in vaccinated sera. Sera from 13 to 17 days after the second dose of the BNT162b2 vaccine were collected from health care workers at the University of Toyama (n = 740). Antibody levels were measured by the anti-receptor binding domain antibody test (anti-RBD test), and neutralization against wild-type (WT), α- and β-variant pseudotyped viruses were assayed using a high-throughput chemiluminescent reduction neutralizing test (htCRNT; positivity cutoff, 50% neutralization at serum dilution 1:100). Basic clinical characteristics were obtained from questionnaires. Antibodies were confirmed in all participants in both the anti-RBD test (median, 2,112 U/ml; interquartile range [IQR], 1,275 to 3,390 U/ml) and the htCRNT against WT (median % inhibition, >99.9; IQR, >99.9 to >99.9). For randomly selected sera (n = 61), 100.0% had positive htCRNT values against the α- and β-derived variants. Among those who answered the questionnaire (n = 237), the values of the anti-RBD test were negatively correlated with age in females (P < 0.01). An age-dependent decline in neutralization was observed against the variants but not against the wild-type virus (wild type, P = 0.09; α, P < 0.01; β, P < 0.01). The neutralizing activity induced by BNT162b2 was obtained not only against the wild-type virus, but also against the variants; however, there was an age-dependent decrease in the latter. Age-related heterogeneity of vaccine-acquired immunity is a concern in preventive strategies in the era dominated by variants. IMPORTANCE Since mRNA vaccines utilize wild-type SARS-CoV-2 spike protein as an antigen, there are potential concerns about acquiring immunity to variants of this virus. The neutralizing activity in BNT162b2-vaccinated individuals was higher against the wild-type virus than against its variants; this effect was more apparent in older age groups. This finding suggests that one of the weaknesses of the mRNA vaccine is the high risk of variant infection in the elderly population. Because the elderly are at a higher risk of SARS-CoV-2 infection, the age-dependent decline of neutralization against viral variants should be considered while planning vaccination programs that include boosters.