Ad26.COV2.S as a Heterologous Booster in Adults After Single- or Two-Dose of Inactivated COVID-19 Vaccine

This is a prospective, multi-center, observer-blind Phase 1/2 adaptive study to assess the safety, reactogenicity, and immunogenicity of a booster dose of Ad26.COV2.S in adults ≥ 18 years of age in Study Part A and Part B. A total of 690 participants will be recruited. If the optional Group (A4) is feasible and enrolled, the total recruitment will be 800 participants. Priority enrolment is given to Groups A1 and A2, followed by Groups A3 and B1. Enrolment of groups are open-label allocation and assessor-masked. The Study is divided into 2 Parts: Part A and B. Study Part A is a prospective, multi-center, assessor-masked Phase 1/2 study to assess a booster dose (3rd dose) of Ad26.COV2.S as an IM injection in the deltoid muscle in adults who have completed the two-dose homologous primary series of inactivated vaccine of Sinovac or Sinopharm, 21 to 35 days apart and who meet eligibility criteria. Participants will receive either the full-dose (5x10\^10 vp) or half-dose (2.5x10\^10 vp) of the study product, and at early (45-75 days) or later (90-240 days) time interval and followed from Visit 1 (V1) to Visit 7 (V7) at 336 days. A total of 580 adult volunteers aged 18 years or older, who have verified vaccination cards with documented completion of homologous primary vaccination series with either two doses of Adsorbed COVID-19 (inactivated) Sinovac (SV) or Sinopharm (SP) vaccine, against original and novel variants of SARS-CoV-2 will be enrolled. A 20% dropout rate is assumed. Enrolment is increased to 360 (300+20% dropout) for the safety assessment, to detect common adverse events with an event rate of 1%. Part B is a prospective, multi-center, open-label, assessor-masked Phase 1/2 heterologous prime-boost study to assess the Ad26.COV2.S (full dose) as the 2nd vaccination in subjects who are documented to all have received the 1st dose of Sinovac or all received the 1st dose of Sinopharm COVID-19 vaccine, with an interval of 28 days or more, followed from Visit 1 (V1) to Visit 7 (V7) at 336 days. Together with the Sponsor, the PIs will decide on the feasibility of enrolment of participants who all have already received 1 dose of Sinopharm or who all have received 1dose of Sinovac, based on current public health policy and vaccine coverage. For safety assessment: Adverse events (AEs) will be systematically collected at all clinic visits. Solicited AEs will be assessed in all subjects immediately (30 minutes) after each injection. Subjects (or with necessary supports) will record solicited AEs daily in the Diary Card for the seven days following injection. If a solicited AE is ongoing at a 7-day post-injection follow-up visit (Visit 2), or occurs after 7 days post-injection, the event will be recorded as AE and continued to be followed as per AE monitoring requirements. Adverse events and special reporting situations, whether serious or non-serious, that are related to study procedures or that are related to non-investigational sponsor products will be reported from the time a signed and dated informed consent form (ICF) is obtained until the end of the study/early withdrawal. All other unsolicited AEs will be reported from the time of vaccination until completion of the participant's last study-related procedure. All AESIs, SAEs, and AEs leading to discontinuation from the study (regardless of the causal relationship) are to be reported from the moment of vaccination until completion of the participant's last study related procedure. For immunological assessment: Primary objectives 1. To assess the IgG immune response against the Spike protein of SARS-CoV-2, measured by ELISA and compared with baseline (pre-boost titer) at Days 28, 84, 168 and 336 following either half dose (2.5x10\^10 virus particles (vp)) or full dose (5x10\^10 vp) of Ad26.COV2.S vaccination at pre-specified time intervals in adults who have already received either one-dose or two-doses of an inactivated COVID-19 vaccine. 2. To assess a subset for functional (neutralizing) humoral immune responses elicited by each of the regimens as measured by pseudovirus neutralization assay, IgG at baseline, Days 28, 84, 168 and 336 following Ad26.COV2.S vaccination. Secondary objective 1\. To assess a subset for functional (neutralizing) humoral immune responses elicited by each of the regimens as measured by microneutralization neutralization assay, IgG at baseline, Days 28, 84, 168 and 336 following Ad26.COV2.S vaccination. Exploratory Objectives 1. To characterize PCR-confirmed COVID-19 breakthrough infections following booster vaccination by assessing anti-S and anti-N IgG. 2. To characterize cellular immune responses including Th1/Th2. 3. To consider statistical tests of noninferiority in comparing the different study arms. 4. To assess Adenovirus 26 neutralizing antibodies at baseline if feasible. Study duration: Subjects will be followed for approximately 336 days following Ad26.COV2.S vaccination. The total study period will be 18 months, including 12-month follow-up period. The Clinical Data Management System, including eCRFs, statistical analysis and data archival are under the responsibility of the center of excellence for Biomedical and Public Health Informatics (BIOPHICS), the center of data management for clinical research at the Faculty of Tropical Medicine, Mahidol University. Based on the final protocol of the study, a comprehensive set of CRFs/eCRFs will be prepared to capture all the relevant data required for analysis and reporting. Information about COVID-19 disease, correlates of immunity, safety, and local epidemiology and public health context regarding the new SARS-CoV-2 virus are rapidly evolving during the pandemic. Therefore, it is critical to recognize that the approach outlined in this document may be adapted as new data, expert consensus and public health policies evolve.