Declining Prevalence of SARS-CoV-2 Antibodies among Vaccinated Nursing Home Residents and Staff Six Months after the Primary BNT162b2 Vaccination Campaign in Belgium: A Prospective Cohort Study

Abstract

In the SCOPE study, we monitored SARS-CoV-2 antibodies in a national sample of residents and staff from Belgian nursing homes. Here, we report the seroprevalence among infected and infection-naive residents and staff after the primary COVID-19 vaccination campaign. Among 1554 vaccinated nursing home residents and 1082 vaccinated staff from 69 nursing homes in Belgium, we assessed the proportion having SARS-CoV-2 antibodies approximately two (April 2021), four (June 2021), and six months (August 2021) after a two-dose regimen of the BNT162b2 vaccine. We measured the seroprevalence using SARS-CoV-2 antibody rapid tests and collected socio-demographic and COVID-19 medical data using an online questionnaire. Two months after vaccination (baseline), we found a seroprevalence of 91% (95% CI: 89-93) among vaccinated residents and 99% (95% CI: 98-99) among vaccinated staff. Six months after vaccination, the seroprevalence significantly decreased to 68% (95% CI: 64-72) among residents and to 89% (95% CI; 86-91) among staff (p < 0.001). The seroprevalence was more likely to decrease among infection-naive residents, older residents, or residents with a high care dependency level. These findings emphasize the need for close monitoring of nursing home residents, as a substantial part of this population fails to mount a persistent antibody response after BNT162b2 vaccination.

Keywords: BNT126b2; COVID-19 vaccination; IgG; IgM; SARS-CoV-2; elderly; nursing home residents; seroprevalence.

Figure 1

Overview of the total number of participants in the cohort and the participation in April, June and August 2021.

Figure 2

Adjusted seroprevalence among nursing home residents (A) and staff (B) with and without self-reported history of SARS-CoV-2 infection, Belgium, two (April 2021), four (June 2021) and six months (August 2021) after COVID-19 vaccination. SARS-CoV-2 seroprevalence was estimated based on a generalized estimating equation model averaged over nursing homes. Blue dots represent the seroprevalence among participants with a self-reported history of SARS-CoV-2 infection; red dots represent the seroprevalence among participants without self-reported history of SARS-CoV-2 infection. The error bars represent the 95% confidence intervals of these means. A history of COVID-19 infection was defined as a previously self-reported positive PCR and/or rapid antigen test, and/or CT-scan between February 2020 and the moment of antibody testing.