Outbreak of SARS-CoV-2 B.1.617.2 (Delta) variant in a Nursing Home 28 weeks after two doses of mRNA anti-Covid-19 vaccines: evidence of a waning immunity

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 48h 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 lower/equal to 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, CI95% 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.