Results
The demographic characteristics of the participants were presented in table 1. Both vaccine boosters produced detectable PRNT50 antibody titers (Figure 2A). Three to five months after two doses of CoronaVac, 40 of 52 (76.92%) participants had neutralizing antibodies above 1/20 with a GMT of 23.27 (range between <1:10 to 1:80). One month after administration, BNT162b2 booster induced significantly higher neutralizing antibody with GMT of 78.69 (ranging between 20 to 1280) compared to GMT of CoronaVac with 21.44 (ranging between 10 to 40; P<0.001). In this period, all participants in the BNT162b2 and 7/10 (70%) of CoronaVac receivers had PRNT50 levels >1/20 titer. The ninety-eight % of the BNT162b2 and 40% of the CoronaVac participants had antibody titers above the 1:30 threshold. In both groups, GMTs remained at similar levels at month three (Figure 2A). The neutralizing antibody levels of each participant in the study period were presented in Figure 2B.
We studied the reactivity and magnitude of CD4+ and CD8+ T cell responses to the SARS-CoV-2 spike S peptide pool. In the assessment of 33/52 patients, there was no significant change in the magnitude of CD4+ and CD8+ T cell ratios compared to baseline levels (Figure 3).
In the ELISpot assay, T cells showed significantly higher IFN-γ reactivity after BNT162b2 booster, with a median of 140 SFU per million PBMC (Range 5 SFU- 320 SFU) compared to baseline T cell response (60 SFU/106 PBMC; p<0.001). The IL-2 reactivity of the T cells, 1 month after BNT162b2 booster (60 SFU/106 PBMC) was significantly higher than CoronaVac booster (median SFU 30 /106 PBMC, p<0.05) and baseline levels (20 SFU/106 PBMC; P<0.001) (Figure 4).
We also assessed the effector T cell response after COVID-19 vaccine booster doses, by phenotyping S-specific CD4+ and CD8+ T cells. Representative gating was shown in figure S1 (supplement). One month after the BNT162b2 booster, both of the CD8+CD38+CD69+ T cells and CD4+CD38+CD69+ T cells were stimulated although not significantly compared to baseline level, however, a significant increase was detected at the 3rd month. After three months, the proportion of CD8+CD38+CD69+ T cells increased from baseline of 1.160 % to 3.130 % (p=0.031) and CD4+CD38+CD69+ T cells from 4.37 % of baseline to 10% (p=0.013). Three months after the CoronaVac booster, CD4+CD38+CD69+ T cells increased from 4.37 % of baseline to 10.20%. (Figure 5).