As older age and having certain comorbidities can influence humoral responses to vaccination, we studied antibody responses after the COVID-19 booster campaigns in nursing home (NH) residents. In a two year longitudinal study with Dutch NH residents (n = 107), aged 50 years and over, we monitored antibody responses in serum prior to and after vaccination with a third, fourth BNT162b2 (wild-type; WT), and a BNT162b2 bivalent (WT/OMI BA.1) fifth vaccine. Data on vaccinations, infections, comorbidities, and, for some participants, clinical symptoms after infection were obtained with questionnaires. Data were compared to antibody responses of BNT162b2-vaccinated, healthier community-dwelling older adults (n = 32) from the general population. The booster vaccinations substantially increased anti-WT and anti-Omicron SARS-CoV-2 Spike S1 (S1) and Spike protein receptor binding domain (RBD)-antibody concentrations of NH residents. This resulted in comparable antibody levels between NH residents and healthier community-dwelling older adults and between infection-naïve and infected NH residents, and in a decline in treatment duration and clinical symptom severity in SARS-CoV-2-infected NH residents. Between one and twelve months after the bivalent fifth dose, anti-Omicron BA.1 antibody levels of the NH residents waned faster than those against the WT strain. The booster vaccinations upheld humoral responses of NH residents to WT and Omicron SARS-CoV-2. This, in addition to the less virulent circulating strains, decreased symptom severity and treatment durations for SARS-CoV-2-infected NH residents. Boosting this vulnerable group should, therefore, be continued to prevent waning of humoral immunity and achieve sufficient protection especially against newly emerging variants of concern.
