# Dynamics of SARS-CoV-2 IgG in Nursing Home Residents in Belgium Throughout Three BNT162b2 Vaccination Rounds: 19-Month Follow-Up

## TL;DR

This study tracked antibody levels in nursing home residents in Belgium over 19 months, showing that boosters significantly increased and sustained immunity, especially in those who initially responded poorly to vaccines.

## Contribution

The study provides detailed 19-month antibody dynamics in nursing home residents after three BNT162b2 vaccination rounds, emphasizing the need for repeated immunization in poor responders.

## Key findings

- Infection-naïve residents had low antibody levels after the primary course but a tenfold increase after the first booster.
- Antibody levels remained stable for nine months post-booster, unlike the rapid decline after the primary vaccination.
- 92% of primary vaccine non-responders developed antibodies after the first booster, though at lower levels than initial responders.

## Abstract

Background/Objectives: This study mapped antibody dynamics across three COVID-19 vaccination rounds (primary course, first, and second booster with BNT162b2) in Belgian nursing home residents (NHRs). Methods: Within a national SARS-CoV-2 serosurveillance study (February 2021–September 2022) across Belgian nursing homes, dried blood spots were collected, on which anti-spike SARS-CoV-2 IgG antibodies were quantified by ELISA in international units/mL (IU/mL). Sociodemographic data were collected at the study start and infection history and vaccination data at each sampling round. Results: Infection-naïve NHRs had low antibody levels after primary course vaccination (geometric mean concentration (GMC) 292 IU/mL, 95% confidence interval (95% CI): 197–432), but increased tenfold after first booster (GMC 2168 IU/mL, 95% CI: 1554–3027). While antibodies among NHRs significantly declined within six months after primary vaccination (p < 0.0001), they remained stable for nine months post-booster (p > 0.05). Among primary vaccine non-responders, 92% (95% CI: 82–97%) developed antibodies after the first booster (GMC 594 IU/mL, 95% CI: 416–849), though tenfold lower than initial responders (GMC 4642 IU/mL, 95% CI: 3577–6022). Conclusions: These findings demonstrate that NHRs require tailored vaccination, prioritizing repeated immunization to improve serological outcomes in poor responders such as infection-naive NHRs. Regular immune monitoring could aid in implementing evidence-based vaccine strategies, ensuring optimal protection for vulnerable populations against SARS-CoV-2 and other infectious threats.