Immunogenicity and adverse events of priming with inactivated whole SARS-CoV-2 vaccine (CoronaVac) followed by boosting the ChAdOx1 nCoV-19 vaccine

Abstract Background Responding to SARS-CoV-2 Delta variants escaped the vaccine-induced immunity and waning immunity from the inactivated whole virus vaccine, Thailand recently proposed a heterologous inactivated whole virus vaccine (CoronaVac) viral vector vaccine (ChAdOx1 nCoV-19) prime-boost vaccine regimen(I/V). This study aims to evaluate the immunogenicity and adverse events of this regimen by comparison with homologous CoronaVac, ChAdOx1 nCoV-19, and convalescent serum. Method Immunogenicity was evaluated by the level of IgG antibodies against the receptor-binding domain of the SARS-CoV-2 spike protein (S1 subunit) (anti-S RBD). At 2 weeks following the second dosage, a selection of random samples was tested for plaque reduction neutralisation (PRNT) and Pseudotype-Based Microneutralization test (PVNT) against SARS-CoV-2 variants of concern (VOCs). The safety profile of heterologous CoronaVac-ChAdOx1 nCoV-19 prime-boost vaccine regimen was described by interviewing at the 1-month visit. Result Between April to August 2021,426 participants were included in the study, with 155 obtaining CoronaVac-ChAdOx1 nCoV-19(I/V),32 obtaining homologous CoronaVac(I/I),47 obtaining homologous ChAdOx1 nCoV-19(V/V),169 with history covid-19 infection. Geometric mean titers (GMTs) of anti-S RBD level in the I/V group compare 2wks and 4 wks (873.9 vs 639,p=0.00114).At 4 wks, GMTs of anti-S RBD level in I/V group was 639, 95% CI 63-726,and natural infection group 177.3, 95% CI 42-221, and V/V group 211.1, 95% CI 77-152, and I/I group 108.2, 95% CI 77-152; all p<0.001).At 2 wks, The GMTs of 50%PRNT of 19 sampling from the I/V group is 434.5, 95% CI 326-579, against wild type and 80.4, 95% CI 56-115, against alpha and 67.4, 95% CI 48-95, against delta and 19.8, 95% CI 14-30, against beta; all p<0.001. At 2 wks, The GMTs of 50%PVNT of 15 sampling from the I/V group is 597.8, 95% CI 368-970, against wild type and 163.9, 95% CI 89-301, against alpha and 157.7, 95% CI 66-378, against delta. The AEs in the I/V schedule were well tolerated and generally unremarkable. Conclusion The I/V vaccination is a mixed regimen that induced higher immunogenicity and shall be considered for responding to Delta Variants when only inactivated whole virus vaccine and viral vector vaccine was available.