Homologous and heterologous boosting of the ChAdOx1-S1-S COVID-19 vaccine with the SCB-2019 vaccine candidate: a randomized, observer-blinded, controlled, phase 2 study

ABSTRACT

BackgroundOngoing outbreaks of COVID-19 are driven by waning immunity following primary immunizations and emergence of new SARS-CoV-2 variants which escape vaccine-induced neutralizing antibodies. It has been suggested that heterologous boosters could enhance and potentially maintain population immunity. MethodsWe assessed immunogenicity and reactogenicity of booster doses of different formulations of alum-adjuvanted SCB-2019 vaccine (9 g SCB-2019 with or without CpG-1018 adjuvant, or 30 g SCB-2019 with CpG-1018) in Brazilian adults primed with ChAdOx1-S vector vaccine. S-protein antibodies and ACE2-binding inhibition were measured by ELISA on Days 1, 15 and 29. Participants self-reported solicited adverse events and reactions. ResultsAll SCB-2019 formulations increased S-protein ELISA antibodies and ACE2 binding inhibition to a greater extent than ChAdOx1-S. After 30 g SCB-2019+CpG+alum titers against wild-type S-protein were significantly higher than after ChAdOx1-S on Days 15 and 29, as were titers of neutralizing antibodies against wild-type strain and Beta, Gamma, Delta, and Omicron variants. Boosting with SCB-2019 or ChAdOx1-S was well tolerated with no vaccine-related serious or severe adverse events. ConclusionsBoosting ChAdOx1-S-primed adults with SCB-2019 induced higher levels of antibodies against a wild-type strain and SARS-CoV-2 variants than a homologous ChAdOx1-S booster, highest responses being with the 30 g SCB-2019+CpG+alum formulation.