Cellular and Humoral Immune Responses and Breakthrough Infections After Two Doses of BNT162b Vaccine in Healthcare Workers (HW) 180 Days After the Second Vaccine Dose.

Background: Immunity and clinical protection induced by mRNA vaccines against SARS-CoV-2 have been shown to decline overtime. To gather information on the immunity profile deemed sufficient in protecting against hospitalization, we tested IgG levels, interferon-gamma (IFN-γ) secretion, and neutralizing antibodies 180 days (d180) after the second shot of BNT162b vaccine, in HW. Methods: A total of 392 subjects were enrolled. All received BioNTech/Pfizer from February 2020 to April 2021. The vaccine-specific humoral response was quantitatively determined by testing for IgG anti-S1 domain of SARS-CoV-spike protein. Live virus microneutralization (MN) was evaluated by an assay performing incubation of serial 2-fold dilution of human serum samples, starting from 1:10 to 1:5120, with an equal volume of Wuhan strain and Delta VOC viral solution and assessing the presence/absence of a cytopathic effect. SARS-CoV-2-spike protein-specific T-cell response was determined by a commercial IFN-γ release assay. Results: In 352 individuals, at d180, IgG levels decreased substantially but no results below the assay's positivity threshold were observed. Overall, 22 naive (8.1%) had values above the highest threshold. Among COVID-naive, the impact of age, which was observed at earlier stages, disappeared at d180, while it remained significant for 81 who had experienced a previous infection. Following the predictive model of protection by Khoury, we transformed the neutralizing titers in IU/ml and used a 54 IU/ml threshold to identify subjects with 50% protective immunity. Overall, live virus MN showed almost all subjects with previous exposure to SARS-CoV-2 neutralized the virus as compared to 33% of naive double-dosed subjects (p < 0.0001). All previously exposed subjects had strong IFN-γ secretion (>200 mIU/ml); among 271 naive, 7 (2.58%) and 17 (6.27%) subjects did not show borderline or strong secretion, respectively. Conclusions: In naive subjects, low IgG titers are relatively long-lasting. Only a third of naive subjects maintain neutralizing responses. After specific stimulation, a very limited number of naive were unable to produce IFN-γ. The results attained in the small group of subjects with breakthrough infection suggest that simultaneous neutralizing antibody titers <20, binding antibody levels/ml <200, and IFN-γ <1,000 mIU/ml in subjects older than 58 may identify at-risk groups.

Early Serological Response to BNT162b2 mRNA Vaccine in Healthcare Workers.

PURPOSE: Clinical significance and durability of serological response after mRNA COVID-19 vaccines is under investigation. Data on early virological response are limited. To iden-tify potential predictors of antibody durability, circulating antibody levels were longitudinally ex-plored in healthcare workers included in a follow-up program for SARS-CoV-2 infection. Meth-ods: Subjects meeting the inclusion criteria signed an informed consent. Serum samples were col-lected at baseline, before the first BNT162b2 vaccine, at days 7, 21, 31, 90, and 180 days after the first dose. Serological evaluation was performed by QuantiVac Euroimmune anti-S1 antibody as-say. Only subjects followed-up until day 90 are here considered. RESULTS: Of 340 taken into consid-eration, 265 subjects were naive, and 75 COVID-19 experienced. The former showed a progres-sive increase in their antibody levels before day 90 decline, while the latter showed antibody levels reaching a plateau at day 7 and slightly declining at day 90. All showed antibody levels higher than the assay cut-off at day 31 and 90. Among naive, 108 had an early response whose predic-tors were younger age and female gender (OR 0.94, 95% CI 0.91-0.96, p < 0.0001; and OR 2.58, 95% CI 1.48-4.51, p = 0.0009). Naive subjects experienced a day 30/90 decline in antibody levels, whereas experienced did not. Early response was an independent predictor of higher day 30/90 antibody levels decline (OR = 2.05, 95% CI 1.04-4.02; p = 0.037). CONCLUSIONS: Our results suggest that in healthcare workers early response might be inversely associated with antibody levels 90 days after BNT162b2 vaccine.