Immunological memory and neutralizing activity to a single dose of COVID-19 vaccine in previously infected individuals

BACKGROUND: The efficacy of COVID-19 vaccines to generate immunological memory post-vaccination has not previously been studied. OBJECTIVE: To assess immunological memory in previously SARS-CoV-2 infected individuals after a single dose of mRNA vaccine. PATIENTS AND METHODS: Healthcare workers (n = 280) were enrolled after obtaining written informed consent and grouped under previously infected and no prior exposure (reverse transcription-polymerase chain reaction positive and negative, respectively). Blood was drawn at baseline and post-vaccination (single dose of COVISHIELD) for enumerating neutralizing antibodies by chemiluminescence and memory T- and B-cells by flow cytometry. RESULTS: Post vaccination, compared with the no prior exposure group, the previously infected group had higher levels of: antibody response (1124.73 ± 869.13 vs 94.23 ± 140.06 AU/ml, p = 0.0001); CD4 memory T-cells: central memory CCR7+CD45RA- (p = 0.0001), effector memory CCR7-/CD45RA- (p = 0.01); total CD8+ T-cells (p = 0.004); CD8+ naïve T-cells CCR7+CD45RA+ (p = 0.01); and memory B-cells CD20+CD27+ (p = 0.0001). DISCUSSION: Single-dose vaccination elicited higher neutralizing antibody response and protective immunity in individuals who had recovered from SARS-CoV-2 infection compared with those with no prior exposure.