Emerging evidence is offering significant insights into the effectiveness and safety of the vaccination against the Coronavirus Disease 2019 (COVID-19), but another crucial aspect of the current global vaccination campaign is the time trend of the antibody response to COVID-19 vaccines over a longer period and the resulting duration of the protection offered.1 Here, we present data on the dynamics of antibodies that bind SARS-CoV-2 spike (S) protein receptor binding domain (RBD)—the most critical target for SARS-CoV-2-specific antibodies within the S1 sub-unit2—after 6 months from the administration with BNT162b2 vaccine.
This analysis, built as a longitudinal observational design, is part of the VASCO project (‘Monitoraggio della risposta al Vaccino Anti-SARS-CoV-2/COVID-19 negli operatori sanitari del Pineta Grande Hospital’), which defines an ongoing broad study on the response to BNT162b2 mRNA COVID-19 vaccine in a sample of healthcare workers (HCWs) of the Pineta Grande Hospital (Castel Volturno, Caserta, Italy), investigating effectiveness, immunogenicity and safety of the vaccination. Complete methods of the VASCO project have been presented elsewhere.3
In this survey, HCWs who were administered the two-dose BNT162b2 mRNA vaccine 21 days apart between December 2020 and January 2021 were invited to undertake a quantitative serology test for the research of SARS-CoV-2 S-RBD-specific immunoglobulins G (IgG). Seroconversion, defined as the development of any detectable SARS-CoV-2 S-RBD-specific IgG in serum sample, was evaluated through Snibe—Maglumi® SARS-CoV-2 S-RBD IgG chemiluminescent immunoassay (CLIA).4 Reactivity was intended as an antibody level equal to or greater than 1.0 AU/ml. According to the manufacturer’s recommendations, the Maglumi® SARS-CoV-2 S-RBD IgG CLIA presented sensitivity of 100% (95% confidence interval [CI], 99.9–100%) and specificity of 99.6% (95% CI, 98.7–100%) after the 15th day from symptom onset.4 HCWs underwent six longitudinal serological assays every 30 days, the first of which was performed within 1 month after completing the vaccination cycle. If a HCW had had a previous infection with SARS-CoV-2 6 months prior to the vaccination or if he/she had contracted the infection after the administration of the first vaccine dose, the cycle was considered complete with a unique dose, as per Italian Ministry of Health guidelines and according to literature findings.5,6 CLIA results were expressed as median IgG value and interquartile range (IQR). Differences between medians were assessed through Mann–Whitney U test; multivariate regression analyses were built to investigate the association between the level of the vaccine-elicited antibodies and potential predictors, such as sex, age, previous SARS-CoV-2 infection and post-first dose infection. A P-value of 0.05 was set as significance level.
Overall, we analyzed the sera of 162 subjects, being mostly women (58.0%) with a mean age of 42.5 years (±11.9 SD). Twenty-eight HCWs had a history of previous SARS-CoV-2 infection. At the first serum sample, the median anti-S-RBD IgG reached 540.0 AU/ml (IQR 64.5–1102.0). In the following tests, a progressive decay of antibodies was seen, up to the value of 55.7 AU/ml (IQR 26.2–84.7) at the 6-month follow-up (Figure 1). No significant associations were found according to vaccinees’ sex and age. Within 1 month from the vaccination, there was a significant higher S-RBD-reactive antibody response in those subjects with previous SARS-CoV-2 infection (medians: 1534.9 [IQR 1142.0–2000.0] vs. 407.7 [IQR 60.0–846.6]; P = 0.001) and the significance remained after adjusting for age and sex (β = 1762.2; 95% CI 1022.9–2501.6; P < 0.001; R2adj = 0.29). Differences in IgG titres between those with previous SARS-CoV-2 infection and those without were no longer significant at the following serological surveys. Three vaccinees tested positive at the RT-PCR assay for qualitative detection of SARS-CoV-2 nucleic acid on nasopharyngeal swabs before the administration of the second vaccine dose; no statistical association was detected between the infection after the first dose and humoral response. No other infections were reported in the follow-up passive surveillance described previously