Objectives The current study was aimed at examining SARS-CoV-2 immune responses following two doses of Comirnaty® COVID-19 vaccine among elderly people in nursing home residences. Methods A prospective cohort study in a representative sample from Valencia nursing homes (n=881; males: 271, females 610; median age, 86 years) was recruited using a random one-stage cluster sampling approach. A lateral flow immunocromatography device (LFIC) (OnSite COVID-19 IgG/IgM Rapid Test; CTK BIOTECH, Poway, CA, USA) was used as the front-line test for detecting SARS-CoV-2-Spike(S)-specific antibodies in whole blood obtained by fingerstick. Residents returning negative LFIC results underwent venipuncture and testing for presence of SARS-CoV-2-S-reactive antibodies and T cells by the Roche Elecsys® Anti-SARS-CoV-2 S (Roche Diagnostics, Pleasanton, CA, USA), the LIAISON® SARS-CoV-2 TrimericS IgG assay (Diasorin S.p.A, Saluggia, Italy), and by flow cytometry, respectively. Results The SARS-CoV-2-S antibody detection rate in nursing home residents was 99.6% (283/289) and 98.3% (587/597) for SARS-CoV-2 recovered and naïve residents, respectively, within a median of 99 days (range, 17-125 days) after full vaccination. Three out of five residents lacking SARS-CoV-2-S antibodies had detectable S-reactive CD8 + and/or CD4 + T cells. 50/50 and 40/50 participants with detectable SARS-CoV-2 antibodies also had SARS-CoV-2-S-reactive IFN-γ-producing CD4 + and CD8 + T cells, respectively. Conclusion The Comirnaty® COVID-19 vaccine is highly immunogenic in nursing home residents.

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