Establishing an objective immune protection threshold is essential to optimize and individualize SARS-CoV-2 vaccination. Using nationwide registries of solid organ recipients, Callaghan et al1 recently confirmed that vaccination reduces hospitalization and death in the Omicron era. Although measurement of binding antibody levels can be used to assess vaccine immunogenicity, such testing has not gained widespread clinical application in part because no threshold value of immune protection has been objectively determined. Immunosuppressed populations provide an opportunity to characterize the protective effect of various antibody levels because of their heterogeneous response to vaccines.
The plaque reduction neutralization test (PRNT) is the gold standard for assessing immune protection.2 It measures the capacity of a neutralizing antibody to block the binding of the virus to the angiotensin-converting enzyme 2 receptor. Because PRNT requires a biosafety level-3 laboratory, the surrogate virus neutralization test (sVNT) has been developed for biosafety level-2 laboratories.3
We analyzed the response to COVID-19 vaccination using an anti-spike SARS-CoV-2 binding antibody quantitative assay (Euroimmun QuantiVac, Germany) and a semiquantitative sVNT (Genscript Biotech, New Jersey). Dilution analyses using the live virus previously established that a 90% reduction of infected cells (PRNT90) correlates with a 30% or more signal inhibition on the sVNT.3 This positive inhibition cutoff defines adequate immune protection.2,4 We first performed sVNT using the horseradish peroxidase wild-type receptor-binding domain protein and then using the Omicron B.1.1.529-BA.1 receptor-binding domain protein (Genscript Biotech, New Jersey), from which the current lineages and sublineages are derived.5–7
Sera from 74 adult kidney transplant recipients were examined after 2 or 3 doses of the vaccine, providing a wide range of binding antibody values (8–3284 binding antibody units [BAU]/mL; mean ± SD, 476 ± 877 BAU/mL; Table S1, SDC, https://links.lww.com/TP/C736). No participant had SARS-CoV-2 infection before sampling. Positive inhibition was observed in 38 and 11 sera for the wild-type and the Omicron protein, respectively (Figure 1). ROC analyses indicated that the binding antibody had a high discriminative ability for positive inhibition (area under the curve 0.99 for the wild type and Omicron, Figure 1). Coordinates of the curves provided positive inhibition thresholds at a binding antibody concentration of 73 BAU/mL (sensitivity, 100%; specificity, 94%) for the wild-type and 752 BAU/mL (sensitivity, 100%; specificity, 98%) for the Omicron variant. Consequently, the 752 BAU/mL threshold resulted in a 100% negative predictive value of immune protection for Omicron, whereas 1357 BAU/mL indicated a positive predictive value of 100% (specificity 100% for neutralization). By univariate logistic regression, a younger age, a higher lymphocyte count, and an absence of mycophenolate in the regimen were a associated with neutralization (Table S2, SDC, https://links.lww.com/TP/C736).
;)
FIGURE 1.: Antibody levels in patients with adequate vs insufficient immune protection based on the sVNT 30% cutoff. A, This shows that immune protection (>30% neutralization on sVNT) for the wild-type protein was observed in 38 of the 74 kidney recipients. The dotted line indicates the threshold value of 73 BAU/mL, established by ROC analysis (sensitivity 100%, specificity 94%). ROC in (B) depicts the diagnostic accuracy of antibody levels to allow discrimination of adequate immune protection for wild-type virus (AUC, 0.99; 95% CI, 0.97-1.00). C, This shows that immune protection (>30% neutralization on sVNT) for the Omicron protein was observed in 11 of the 74 kidney recipients. The dotted lines indicate the threshold value of 752 BAU/mL (sensitivity 100%, specificity 98%) and 1357 BAU/mL (sensitivity 82%, specificity 100%), as determined by ROC analysis. ROC in (D) depicts the diagnostic accuracy of antibody levels to allow discrimination of adequate immune protection for Omicron (AUC, 0.99; 95% CI, 0.99-1.00). AUC, area under the curve; BAU, binding antibody unit; CI, confidence interval; ROC, receiver operator characteristic; sVNT, surrogate virus neutralization test.
Karaba et al8 recently reported poor neutralization against Omicron in transplant recipients. The difference with our results may be due to a smaller cohort of kidney recipients, 16 patients, which lowered their likelihood to identify patients who neutralized Omicron.
These data suggest that results <752 and >1357 BAU/mL, by serology, identify individuals who, respectively, fail and succeed to neutralize the Omicron variant. Given the clinical vaccine effectiveness with the Omicron variant,1 such information could guide the timing of booster dose administration according to each individual’s immune response.
ACKNOWLEDGMENTS
The authors thank all the individuals who participated in the study. They thank France Samson, Annie Saillant, and Marie-Pier Roussel for their help with patient recruitment and sample collection.
REFERENCES
1. Callaghan CJ, Curtis RMK, Mumford L, et al. Vaccine effectiveness against the SARS-CoV-2 B.1.1.529 Omicron variant in solid organ and islet transplant recipients in England: a national retrospective cohort study. Transplantation. 2023;107:1124–1135.
2. Garcia-Beltran WF, Lam EC, Astudillo MG, et al. COVID-19-neutralizing antibodies predict disease severity and survival. Cell. 2021;184:476–488.e11.
3. Tan CW, Chia WN, Qin X, et al. A SARS-CoV-2 surrogate virus neutralization test based on antibody-mediated blockage of ACE2–spike protein–protein interaction. Nat Biotechnol. 2020;38:1073–1078.
4. Taylor SC, Hurst B, Charlton CL, et al. A new SARS-CoV-2 dual-purpose serology test: highly accurate infection tracing and neutralizing antibody response detection. J Clin Microbiol. 2021;59:e02438–e02420.
5. Centers for Disease Control and Prevention. Summary of variant surveillance. Available at
https://covid.cdc.gov/covid-data-tracker/#variant-summary. Accessed February 9, 2023.
6. Centers for Disease Control and Prevention. COVID data tracker weekly review. Monitoring variants. Available at
https://www.cdc.gov/coronavirus/2019-ncov/covid-data/covidview/past-reports/10212022.html. Accessed February 9, 2023.
7. Centers for Disease Control and Prevention. Morbidity and mortality weekly report (MMWR). Available at
https://www.cdc.gov/mmwr/volumes/72/wr/mm7205e1.htm?s_cid=mm7205e1_w. Accessed February 9, 2023.
8. Karaba AH, Johnston TS, Aytenfisu TY, et al. A fourth dose of COVID-19 vaccine does not induce neutralization of the Omicron variant among solid organ transplant recipients with suboptimal vaccine response. Transplantation. 2022;106:1440–1444.
