Background
Nursing Home (NH) residents are at high risk of serious illness and death from COVID-19 due to Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) 1. In NHs facing a COVID-19 outbreak, infection prevention and control (IPC) measures based on results of repeated testing by real-time reverse-transcriptase polymerase chain reaction for SARS-CoV-2 (RT-PCR) in all residents can limit coronavirus transmission 2,3. Residents with a prior COVID-19 may be protected against reinfection during outbreaks in NHs 4.
Vaccination against SARS-CoV-2 is safe and effective in preventing COVID-19 in adults 5, and immunocompetent SARS-CoV-2 seropositive adults might only require one dose of the Pfizer-BioNTech or the Moderna mRNA vaccines to reach immunity 6,7.
Age or immune dysregulation may impact immune response to COVID-19 and vaccination. The level of N-protein IgG produced after SARS-CoV-2 infection, associated with a substantially reduced risk of SARS-CoV-2 reinfection 8, declines over time 9. In the Danish Microbiology Database, protection against repeat infection was found to be reduced in participants over 65 years10. Age seems also to reduce serological response after vaccination of kidney 11 or liver transplant recipients 12, and of patients with chronic lymphocytic leukemia, 13 cancer 14or hemodialysis. 15
Few data exist on post-vaccine immunity in NH residents, and the size of the study is usually low. After one jab, binding antibodies against S-protein were approximately 4-fold lower in residents compared to healthcare professionals. 16,17 Although the increment of antibody levels after the second immunisation may be higher in older participants, the absolute mean titer of this group seems to remain lower in NH residents aged over 80 years than in those under 60.18
A pilot study showed that NH residents having had COVID-19 in the past 9-12 months had much higher levels of antibodies against the SARS-CoV-2 S-protein (S-protein IgG) after a single BNT162b2 jab than residents not having had COVID-19 16.
It remains therefore necessary i) to confirm in a large sample of residents whether the S-protein IgG response after a single BNT162b2 jab may be influenced by prior COVID-19 infection, ii) to determine in former COVID-19 residents whether the S-protein IgG response after a single BNT162b2 jab may be influenced by the time elapsed since the COVID-19 infection and by the persistence of a natural immunity against N-protein and (iii) to assess the S-protein IgG response after a second vaccination in residents with or without prior COVID-19.
We first compared S- and N-protein IgG levels 3 weeks after a single BNT162b2 jab in a large group of residents with vs without prior COVID-19 confirmed by RT-PCR (RT-PCR-positive residents). Two periods were considered - the past 9 to 12 months (older infections) and the past 3 to 7 months (newer infections) - corresponding to the first and second waves of the epidemic in our region (March-June 2020 and September-December 2020). Secondly, we assessed the S-protein IgG response in these residents 6 weeks after a second vaccination.