Strikingly, however, the breakthrough infection was able to elicit cross-reactive broadly neutralising antibodies (bNAbs) from the unmeasurable level (<1:20) to the mean IC50 value of 1:2929 (range 588.5–5508) at 9 days post symptoms onset (PSO) in OP1 and from the mean IC50 value of 1:24.3 to 1:854.5 at 12 days PSO in OP2, respectively (Figure 1D). ...the amounts of NAbs in OP1 and OP2 were consistently higher than the mean IC50 values of BNT162b2 vaccinees across all VOCs tested (Figure 1B). In particular, there were 121.41- and 74.89-fold higher IC50 values against beta and omicron in OP1 than those in BNT162b2 vaccinees (Figure 1B). Besides NAbs against the current panel of VOCs, OP1 also displayed enhanced IC50 values of NAbs against 15/16 SARS-CoV-2 variants with individual mutations or deletions, including the E484K mutation, which conferred significant resistance to vaccine-induced NAbs (Figure S1). Spike- and NP-specific CD8 IFN-γ responses were 0.56% and 0.11% in OP1 and 0.10% and 0.08% in OP2, respectively. ...the Spike-specific CD4 and CD8 T-cell responses were relatively higher in OP1 or comparable in OP2 as compared with mean values in BNT162b2 vaccinees (CD4 T: mean 0.19% and CD8 T: mean 0.10%).