Safety and immunogenicity of UB-612 heterologous booster in adults primed with mRNA, adenovirus, or inactivated COVID-19 vaccines: a randomized, active-controlled, Phase 3 trial.

Background: Authorized COVID-19 vaccines require boosters for continued protection; however, the lack of cross-platform compatible boosters creates practical challenges to keeping populations protected. Methods: This Phase 3, multicenter, international, randomized, active-controlled trial compared UB-612 as a third-dose heterologous booster to BNT162b2, ChAdOx1-S, or BBIBP-CorV homologous boosters in healthy subjects aged ≥16 years. Participants were randomly assigned 1:1 to receive a single intramuscular injection of UB-612 or an active comparator matching the primary dose, and were stratified for age, sex, N-protein seropositivity, and time since the last dose of their primary series COVID-19 vaccination. The primary objective was to show non-inferiority of neutralizing antibody geometric mean titer (GMT) against live SARS-CoV-2 Wuhan strain after boosting with UB-612 or each of the licensed platform vaccines. Secondary and exploratory objectives covered short and long-term antibody responses. The safety analysis addressed subject and investigator reported adverse events. The study was registered on ClinicalTrials.gov, NCT05293665, and completed on September 12, 2023. Findings: Between March 22 and September 9, 2022, 469 subjects received UB-612 as a heterologous booster, and 467 received BNT162b2 (n = 204), ChAdOx1-S (n = 95), or BBIBP-CorV (n = 168) as homologous boosters. Over 90% of all subjects were positive for N-protein antibody at baseline. When compared to the respective homologous booster response, UB-612 stimulated Wuhan and Omicron BA.5 neutralizing antibody responses that were non-inferior, thus meeting all primary and secondary immunogenicity endpoints of the study. Importantly, UB-612 demonstrated superiority in neutralizing antibody GMT and seroresponse rates compared to ChAdOx1-S and BBIBP-CorV. UB-612 was also effective in stimulating neutralizing antibodies against a more recent Omicron XBB1.5 strain. Long-term immunity analysis through 6- and 12-month follow-ups favored UB-612 over ChAdOx1-S and BBIBP-CorV and supported comparable immunity to BNT162b2. All vaccines were well tolerated and had similar safety profiles. Interpretation: In a pivotal Phase 3 study, UB-612 demonstrated the potential for broad use as a cross-platform heterologous booster, restoring protective immunity in adults previously vaccinated with mRNA, adenovirus-vectored, or inactivated virus-based COVID-19 vaccines. Funding: The study was co-funded by the Coalition for Epidemic Preparedness Innovations (CEPI) and Vaxxinity.