ABSTRACT
The ongoing COVID-19 pandemic has hit long-term care facilities (LTCF), with outbreaks affecting both residents and health care workers (HCWs). Elderly persons have been prioritized in the implementation of vaccination programs. Here we investigated a COVID-19 outbreak, caused by the Beta variant (B.1.351) in a LTCF where residents and HCWs had received 2 doses of Comirnaty vaccine (Pfizer/BioNTech) until one month before the outbreak. Samples from 14 residents (SARS-CoV-2 PCR-negative: n = 8, PCR-positive: n = 6) and 10 HCWs (PCR-negative: n = 10) were collected at a median of 54 days following the second vaccine dose. IgG antibodies to SARS-CoV-2 spike glycoprotein and neutralizing antibody (NAb) titers were measured. Additionally, functional responses of PBMCs to SARS-CoV-2 spike and nucleocapsid proteins were investigated. We observed that Comirnaty induced higher IgG concentrations and NAb titers in HCWs compared to residents. PBMCs of HCWs responded vigorously to stimulation with SARS-CoV-2 spike glycoprotein, with the secretion of interferon gamma, granzyme B and perforin-1 into supernatants. In comparison, only 3 of 9 samples from residents showed positive cellular responses to spike glycoprotein. Group-level cellular responses directed at SARS-CoV-2 nucleoprotein remained low both in HCWs and in residents. Only 2 of 2 PCR-positive residents showed a positive response consistent with exposure to SARS-CoV-2 breakthrough infection. Our results show that elderly persons are at increased risk for breakthrough infection after vaccination. Weak vaccine-directed responses in the elderly need to be addressed in vaccination protocols.