Objectives To measure the antibody decay after two BNT162b2 doses and the antibody response after a third vaccine dose administered 6 months after the second one in nursing home residents with and without prior COVID-19. Design Cohort study. Setting and Participants 418 residents from 18 nursing homes. Methods Blood RBD-IgG (IgG II Quant assay, Abbott Diagnostics; upper limit: 5680 BAU) and nucleocapsid-IgG (Abbott Alinity) were measured 21-28 days after the second BNT162b2 dose, as well as 1-3 days before and 21-28 days after the third vaccine dose. RBD-IgG levels ≥ 592 BAU/mL were considered as high antibody response. Residents with prior positive RT-qPCR on a nasopharyngeal swab or with N-IgG levels above 0.8 S/CO were considered as prior COVID-19 residents. Results In prior COVID-19 residents (n=122), RBD-IgG median levels decreased by 82% in 167 days on average. In the same period, the number of residents with a high antibody response decreased from 88.5% to 54.9% (p<.0001) and increased to 97.5% after the third vaccine dose (p=0.02 vs the first measure). In residents without prior COVID-19 (n=296), RBD-IgG median levels decreased by 89% in 171 days on average. The number of residents with a high antibody response decreased from 29.4 to 1.7% (p<.0001) and increased to 88.4% after the third vaccine dose (p<.0001 vs the first measure) Conclusions and Implications The strong and rapid decay of RBD-IgG levels after the second BNT162b2 dose in all residents and the high antibody response after the third dose validate the recommendation of a third vaccine dose in residents less than 6 months after the second dose, prioritizing residents without prior COVID-19. The slope of RBD-IgG decay after the third BNT162b2 dose and the protection level against SARS-CoV-2 B.1.1.529 (omicron) and other variants of concern provided by the high post-boost vaccination RBD-IgG response require further investigation in residents.
Strong decay of SARS-CoV-2 spike antibodies after two BNT162b2 vaccine doses and high antibody response to a third dose in nursing home residents