ABSTRACT
Introduction: This phase I study explored the immunogenicity and reactogenicity of accelerated, Q7 fractional, intradermal vaccination regimens for COVID-19. Methods: Participants (n = 60) aged 18-60 years, naïve to SARS-CoV-2 infection or vaccination, were randomly allocated into one of four homologous or heterologous accelerated two-dose, two-injection intradermal regimens seven days apart:(1) BNT162b2-BNT162b2(n= 20),(2) ChAdOx1- BNT162b2 (n = 20), (3) CoronaVac-ChAdOx1 (n = 10), and (4) ChAdOx1-ChAdOx1 (n = 10). CoronaVac and ChAdOx1 were 20%, and BNT162b2 17%, of their standard intramuscular doses (0.1 mL and 0.05 mL per injection, respectively). Humoral immune responses were measured through IgG response towards receptor binding domains (RBD-IgG) of ancestral SARS-CoV-2 spike protein and pseudovirus neutralization tests (PVNT50). Cellular immune responses were measured using ELISpot for ancestral protein pools. Results: Immunogenicity was highest in regimen (2), followed by (1), (4), and (3) 2 weeks after the second dose (P < 0.001 for anti-RBD-IgG and P= 0.01 for PVNT50). Each group had significantly lower anti-RBD IgG (by factors of 5.4, 3.6, 11.6, and 2.0 for regimens (1) to (4), respectively) compared to their respective standard intramuscular regimens (P < 0.001 for each). Seroconversion rates for PVNT50 against the ancestral strain were 75%, 90%, 57% and 37% for regimens (1) to (4), respectively. All participants elicited ELISpot response to S-protein after vaccination. Adverse events were reportedly mild or moderate across cohorts. Discussion: We concluded that accelerated, fractional, heterologous or homologous intradermal vaccination regimens of BNT162b2 and ChAdOx1 were well tolerated, provided rapid immune priming against SARS-CoV-2, and may prove useful for containing future outbreaks.