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Objectives: Description of a SARS-CoV-2 B.1.617.2 (Delta) variant outbreak among residents (N = 69) and Health Workers (HWs: N = 69) of a small Nursing Home in Northern-East Italy, with full vaccination coverage of 91 and 82 %, respectively. Evaluation of the Anti-Spike IgG titers 28 weeks after the mRNA vaccine boosts against SARS-COV-2 infection and severe Covid-19. Materials and methods: A timely collection of sera within 48 h from the index case; anti-Spike IgG determination (expressed as Binding Antibody Units – BAU/mL) through a commercial quantitative assay; SARS-CoV-2 diagnostics via RT-PCR, and full-genome sequencing for lineage characterization. Residents were grouped according to anti-Spike IgG titers (≤50, 51-1000, and >1000 BAU/mL) and resulting protection against the infection and the severe disease was measured. Results: 0/20 HWs and 14/59 (24 %) residents fully vaccinated and without a previous SARS-CoV-2 infection showed anti-Spike IgG ≤50 BAU/mL (1-sided Fisher exact p=0.011). Among these residents, a level of anti-Spike IgG ≤50 BAU/mL resulted in a higher risk of SARS-CoV-2 infection (RR=1.55, CI 95% 1.17-2.05) and severe Covid-19 disease (RR=5.33, CI95% 1.83-15.57). Conclusion Low levels of SARS-CoV-2 neutralizing anti-Spike IgG in serum 28 weeks after the administration of the second dose parallels the waning of vaccine protection.

Competing Interest Statement

The authors have declared no competing interest.

Funding Statement

This study did not receive any funding

Author Declarations

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.


The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

The study was approved by the competent ULSS 7 Bassano del Grappa (VI, Italy) Ethics Committee on Aug 26, 2021.

I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.


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Paper in collection COVID-19 SARS-CoV-2 preprints from medRxiv and bioRxiv

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