Immunogenicity and reactogenicity against the SARS-CoV-2 variants following heterologous primary series involving CoronaVac and ChAdOx1 and BNT162b2 plus heterologous BNT162b2 booster vaccination: An open-label randomized study in healthy Thai adults.

We evaluated the immunogenicity and reactogenicity of heterologous COVID-19 primary series vaccination schedules. Participants were randomized to one of seven groups that received two-dose homologous BNT162b2 or heterologous combinations of CoronaVac, ChAdOx1 and BNT162b2, with 4 weeks interval. Of 210 participants, median age was 38 (19-60) years, 51% were female. The groups that received BNT162b2 as second dose induced the highest virus-specific IgG response against the ancestral strain [BNT162b2: geometric mean concentration (GMC) 2133-2249, 95%CI 1558 to 3055; ChAdOx1: 851-1201, 95%CI 649 to 1522; CoronaVac: 137-225, 95%CI 103-286 BAU/mL], neutralising antibodies (NAb) against Beta and Delta, and interferon gamma response. All groups induced low to negligible NAb against Omicron. A BNT162b2 booster (3rd dose) following heterologous CoronaVac and ChAdOx1 regimens induced >140-fold increase in NAb titres against Omicron. Our findings indicate that heterologous regimens using BNT162b2 as the second dose may be considered an alternative schedule to maximize immune response.

The immunogenicity and safety of different COVID-19 booster vaccination following CoronaVac or ChAdOx1 nCoV-19 primary series

The CoronaVac (Sinovac Biotech) and ChAdOx1(Oxford-AstraZeneca) are two widely used COVID-19 vaccines. We examined the immunogenicity of four COVID-19 booster vaccine: BBIBP-CorV (Sinopharm Biotech), ChAdOx1, 30g-BNT162b2 and 15g-BNT162b2 (Pfizer-BioNTech), in healthy adults who received a two-dose CoronaVac or ChAdOx1 8-12 weeks earlier. Among the 352 participants (179 CoronaVac and 173 ChAdOx1 participants), 285 (81%) were female, and median age was 39(IQR: 31-47) years. 98%(175/179) and 99%(172/173) of Coronavac and ChAdOx1 participants remained seropositive at baseline. Two weeks post-booster, both 30g- and 15g-BNT162b2 induced the highest anti-RBD IgG concentration (BAU/mL); Coronavac-prime: 30g-BNT162b2, 5152.2(95%CI 4491.7-5909.8); 15g-BNT162b2, 3981.1(3397.2-4665.4); ChAdOx1, 1358.0(1141.8-1615.1); BBIBP-CorV, 154.6(92.11-259.47); ChAdOx1-prime: 30g-BNT162b2, 2363.8(2005.6-2786.1; 15g-BNT162b2, 1961.9(1624.6-2369.1); ChAdOx1, 246.4(199.6-304.2); BBIBP-CorV, 128.1(93.5-175.4). Similarly, both 30g- and 15g-BNT162b2 boosting induced the highest neutralizing antibodies (nAb) titres against all variants and highest T-cell response evaluated by interferon gamma released asssays. While all BNT162b2 or heterologous ChAdOx1-boosted participants had nAb against Omicron, these were <50% for BBIBP-CorV and 75% for homologous ChAdOx1-boosted participants. There was significant decrease in nAb (>4-fold) 16-20 weeks post booster. Heterologous boosting with BNT162b2 following CoronaVac or ChAdOx1 primary series is most immunogenic. A lower dose BNT162b2 may be considered as booster in settings with limited vaccine supply.