Chimpanzee Adenovirus and Self-Amplifying mRNA Prime-Boost Prophylactic Vaccines Against SARS-CoV-2 in Healthy Adults

This is a multicenter, US-only, phase 1, open-label, dose escalation, non-randomized study of the safety, tolerability, and immunogenicity of investigational ChAd and SAM SARS-CoV-2 vaccines in healthy adult subjects. Homologous and heterologous prime-boost vaccination schedules (Stage 1), as well as boost(s) after receipt of COVID-19 EUA/licensed vaccines (Stage 2) will be examined. Subjects' willingness to receive ChAd vaccines will be assessed and documented at the time of informed consent and considered to determine group assignments. This phase 1 study will enroll 17 Stage 1 and up to 118 Stage 2 subjects. Eligible subjects will be enrolled in different groups based on their age (18-60 years old and \>60 years old) and their EUA/licensed COVID-19 vaccination status. A sentinel approach with 72-hour (Stage 1, and Stage 2, Groups 5, 6, 8-10, 12, 13-15) or 7-day observation times (Groups 7 and 11) will be used, before recruiting the remainder of each dose escalation group. Decisions about dose escalation will be determined by the SSC with consultation with the DSMB as needed after all subjects in each group have been observed through Day 8 post first study vaccination. All subjects will be followed through 12 months after their last study vaccination. Vaccinated subjects will be carefully monitored for exposure and infection to SARS-CoV-2 throughout the study. Escalation to the highest dose (10 µg) of SAM-S-TCE in younger subjects will proceed only following safety assessments of the 10 µg dose in older subjects for a period of 28 days post-vaccination. In addition, the dosage of SAM-S-TCE given as a double boost to subjects previously vaccinated with the Johnson \& Johnson/Janssen Ad26 COVID-19 EUA/licensed vaccine in Groups 8A, 8B, and 12A, 12B will be determined based on the dose escalation reactogenicity and immunogenicity results in Groups 5-7 and 9-11, respectively. After protocol version 9.0 was implemented, it was decided not to enroll subjects into Groups 7 and 8 because of competing priorities and predicted difficulties enrolling into these two groups. The primary objectives of this study are 1) To assess the safety and tolerability of different doses of ChAd-S or ChAd-S-TCE, and SAM-S or SAM-S-TCE when administered as prime-boost in healthy naïve adult subjects, 2) To assess the safety and tolerability of different doses of ChAd-S or ChAd-S-TCE, and SAM-S or SAM-S-TCE when administered as first or second boost in healthy adult subjects previously vaccinated with an mRNA or adenoviral-vectored COVID-19 EUA/licensed vaccine. The secondary objective of this study is to assess the humoral immunogenicity of ChAd-S or ChAd-S-TCE, and SAM-S or SAM-S-TCE.