key: cord-0770446-179rogbf
authors: Cassaniti, Irene; Percivalle, Elena; Sarasini, Antonella; Cambiè, Giuseppe; Nepita, Edoardo Vecchio; Maserati, Roberta; Ferrari, Alessandro; Corcione, Alfonso; Di Martino, Raffaella; Bonetti, Alice; Di Napoli, Annapia; Ferrari, Guglielmo; Baldanti, Fausto
title: Seroprevalence of SARS-CoV-2 in 1922 blood donors from the Lodi Red Zone and adjacent Lodi metropolitan and suburban area
date: 2021-03-04
journal: Clin Microbiol Infect
DOI: 10.1016/j.cmi.2021.01.030
sha: 486af35aeaa5161c35ebd9fce25074fafcf7b17a
doc_id: 770446
cord_uid: 179rogbf

OBJECTIVES: To define the seroprevalence of SARS-CoV-2 in blood donors (referred to the first lockdown area (Lodi Red-Zone) of the Lombardy region and in a contiguous area that was not included in the first lockdown ii) to define the agreement of commercial serological assay with a reference microneutralization assay; iii) to evaluate the persistence of SARS-CoV-2 neutralising antibodies in a blood donors cohort. METHODS: Blood donors referred to the first lockdown area in Lombardy Region and the neighbouring area were analysed for SARS-CoV-2 IgG specific antibodies during the period 18March-24June. Serum samples were analysed using both chemioluminescent assay (Liason SARS-CoV-2 S1/S2 IgG, Diasorin) for the quantitative characterization of SARS-CoV-2 anti-S1 and anti-S2 IgG antibodies and neutralising antibodies (NT-Abs) assay. RESULTS: In the period 18 Mach-24 June, 1922 BD were tested for the presence of SARS-CoV2 IgG showing a prevalence of 378/1922 (19.7%). A subgroup of 1139 BD was tested in parallel with SARS-CoV2 IgG assay and a microneutralization assay showing a prevalence of 22.2 and 21.6%, respectively. SARS-CoV2 IgG quantification was correlated with NT-Abs titres. In 78.2% subjects NT-Abs titre was maintained, while in 15.8% decreased of one four-fold dilution and in 6.0% increased of one four-fold dilution. CONCLUSIONS: The duration of immunity of SARS-CoV-2 is crucial for the course of pandemic and for this reason monitoring of NT Abs is important. Despite a stable NT titer was observed in the majority of blood donors, our findings need to be validated in a long-time period of follow-up.

Objectives: To define the seroprevalence of SARS-CoV-2 in blood donors (referred to the first 22 lockdown area (Lodi Red-Zone) of the Lombardy region and in a contiguous area that was not 23 included in the first lockdown ii) to define the agreement of commercial serological assay with a 24 reference microneutralization assay; iii) to evaluate the persistence of SARS-CoV-2 neutralising 25 antibodies in a blood donors cohort.

Methods: Blood donors referred to the first lockdown area in Lombardy Region and the 27 neighbouring area were analysed for SARS-CoV-2 IgG specific antibodies during the period 28 18March-24June. Serum samples were analysed using both chemioluminescent assay (Liason 29 SARS-CoV-2 S1/S2 IgG, Diasorin) for the quantitative characterization of SARS-CoV-2 anti-S1 30 and anti-S2 IgG antibodies and neutralising antibodies (NT-Abs) assay. Results from serologic assays may be slight different according to the type of protein used as 53 antigen. A degree of cross-reactivity was shown in assays that used whole virions and N protein as 54 target for antibodies detection [3] . In contrast, the spike protein (S2) seems to be more specific, including Lodi metropolitan and suburban area agreed to participate to the study and were analysed 77 for SARS-CoV-2 IgG specific antibodies during the period 18March-24June.

The study was approved by the Institutional Review Board of the Fondazione IRCCS QuantitativeSARS-CoV-2 S1/S2 IgG and NT-Abs measurement 82 Serum samples were analysed using chemioluminescent assay (Liason SARS-CoV-2 S1/S2 IgG,

Diasorin) for the quantitative characterization of SARS-CoV-2 anti-S1 and anti-S2 IgG antibodies, 84 according to manufacturer's instructions. Results were given as AU/mL and a cut-off of 15 AU/mL 85 was considered for definition of positive samples. Results ranging from 12 and 15 AU/mL were 86 considered borderline or weak positive while IgG titre less than 12 AU/mL was given as negative 87 result. Reagents were kindly provided by the manufacturer free of charge. Neutralising antibodies 88 (NT-Abs) titre against SARS-CoV-2 was defined as previously described [ 8, 9] . Briefly, 50 µl of 89 sample from each patient, starting from 1:10 in a serial four fold dilution series, were added in two 90 wells of a flat bottom tissue culture microtiter plate (COSTAR,Corning Incorporated, NY 14831, 91 USA), mixed with an equal volume of 100 TCID50 of a SARS-CoV2strain isolated from a 92 symptomatic patient, previously titrated and incubated at 33°C in 5% CO 2 . All dilutions were made AU/mL) and only 4/45 (8.9%) showed high level of S1/S2 IgG (higher than 80 AU/mL).

On the other side, 886/1139 (77.8%) were negative for S1/S2 IgG: of them 847/886 (95.6%) 127 were confirmed as negative by the micro-neutralization assay (NT titre <1:10) while the remaining 128 39/886 (4.4%) tested positive for SARS-CoV-2 NT Abs (NT titre ≥1:10) ( Table 1) Table 1 . Agreement between SARS-CoV-2 NT Abs and SARS-CoV-2 S1/S2 IgG in 1139 134 blood donors. positive  negative  total  positive  208  39  247  negative  45  847  892  Total  253  886  1139  136 SARS-CoV-2 S1/S2 IgG level was related to SARS-CoV-2 NT Abs titre 137 We analysed the S1/S2 IgG levels in 247 SARS-CoV-2 NT-Abs positive subjects, stratified tested positive for SARS-CoV-2 S1/S2 IgG. Moreover, we observed a high level of agreement 174 between LIAISON CLIA assay detecting SARS-CoV-2 S1/S2 and the SARS-CoV-2 NT assay. 175 Finally, in a small subset of donors we were able to monitor SARS-CoV-2 NT Abs over time, 176 finding a stable SARS-CoV-2 NT titre in about 78% of donors. 

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