An Experimental Medicine Decipher of a Minimum Correlate of Cellular Immunity

Criteria for Recruitment and Recruitment Process: Subjects will be recruited from SingHealth Investigational Medicine Unit (IMU) healthy volunteer database and recruitment posters. Subjects will be given a copy of the Participant Information and Informed Consent Form to read upon their arrival. A briefing session on the study will be conducted by the Investigator. Thereafter, subjects will be ushered into a private room where informed consent is obtained and where questions about the study can be asked freely. Subjects will not be rushed into making a decision to participate in the study. They will be encouraged to speak to their family members about participation in the study; and allowed to defer their decision (without any prejudice) to participate till after discussion with their family members.

Screening Visits and Procedures: Subjects will be recruited via the SingHealth Investigational Medicine Unit (IMU). Informed written consent will be sought from subjects who fulfill criteria for enrollment. All consented subjects will undergo screening, which includes physical examination, full blood count, liver function test, anti-dengue antibodies ELISA (Enzyme-Linked Immunosorbent Assay) and urinary pregnancy test (for female subjects of child-bearing potential).

A urine pregnancy test will be performed at screening and on the day of vaccination (day 0 and day 28) for female subjects of child-bearing potential. Only those with a negative urine pregnancy test will be considered to be eligible for the study, provided that other eligibility criteria were fulfilled.

Both male (if he has a partner of childbearing potential) and female subjects (of childbearing potential) must agree to use adequate and reliable contraceptive measures (e.g. spermicides, condoms, contraceptive pills) or practice abstinence for 10 days after vaccination.

Study Visits and Procedures: 58 subjects will be randomised into 1 of 2 arms (Arm B1 and Arm B2) in a 1:1 ratio, in a double-blind fashion. Subjects in Arm B1 will receive JE-YF17D vaccine (Imojev, Sanofi Pasteur) on Day 0 followed by YF17D vaccine (Stamaril, Sanofi Pasteur) on Day 28. Subjects in Arm B2 will receive Stamaril on Day 0 followed by Imojev on Day 28. Arm B3 will be conducted as a separate single-arm open label design in 14 subjects. Subjects in Arm B3 will receive inactivated JE vaccine (Ixiaro, Valneva) on Day 0 followed by Stamaril on Day 28. Study visits in all arms will occur on Day 0, 4, 7, 10, 14, 28, 29, 32, 35, 38, 42, 58. At each study visit, physical examination, vital signs and research blood sampling will be performed. On Day 0 and Day 28, blood sampling will be performed prior to vaccination.

Final study visit: Last study visit will be on Day 58 post-study vaccination.

Post study follow up and procedures: There is no requirement for post-study follow-up or procedures.

Safety Monitoring Plan: The study may be evaluated by government inspectors/ regulatory authorities who must be allowed access to e-CRFs, source documents, and other study files. The inspectors will review CRFs and compare them with source documents to verify accurate and complete collection of data and confirm that the study is being conducted according to the protocol, ICH Good Clinical Practices (ICH-GCP) and all applicable regulations.

At any time post-vaccination, all subjects will be trained to observe for systemic AEs. A diary will also be given to the subjects to record such events should they occur during this period. Should they develop systemic symptoms that require intervention, they will report to the study site for medical evaluation and receive the appropriate therapy. Duration of symptoms will be recorded. Any concomitant medication use during this period will also be recorded.

During the study, full history taking and physical examination will be performed at both scheduled and unscheduled visits. A full physical examination will only be done at the screening visit, while a brief physical examination will be done for all subsequent visits. The Common Terminology Criteria for Adverse Events (CTCAE), routinely employed in clinical trials, will be used to define AE terminology and severity. Management of AEs is at the discretion of the study team PI and co-Is, guided by severity and clinical indication for intervention. All medication prescribed for the management of AEs will be documented in the medication/concomitant medication clinical record form. Details of AE event terminology, date and time of event start and end, severity, using the CTCAE or treatment given, impact on work and to the continuation of the study, and final outcome of the event will be recorded on the case report until resolution of the event.

Data Quality Assurance: The PI and Co-Is will review the study periodically for data and safety monitoring. Internal quality checks will be performed by two CRCs who are study team members. The data entered by one CRC will be checked by another using the source documents. The study may also be picked for monitoring by SingHealth Office of Research Integrity and Compliance (ORIC) or evaluated by government inspectors/regulatory authorities who must be allowed access to e-CRFs, source documents, and other study files. The monitors/inspectors may review CRFs and compare them with source documents to verify accurate and complete collection of data and confirm that the study is being conducted according to the protocol, ICH-Good Clinical Practices (ICH-GCP) and all applicable regulations.

Data Entry and Storage: All participant's data will be de-identified upon recruitment. Hardcopy research data collection forms such as CRFs, logs and diaries will be kept in the Investigator's Site File and stored in SingHealth IMU under lock and key, accessible only to delegated study team members.

Direct data capture of demographic and clinical data will be captured on source documents. Identifiers will be kept in a separate file in another office and every effort will be made to protect the privacy of the participants. The data to be analysed will contain only de-identified data. An electronic data capture system will be used. All electronic data will be password protected and can only be accessed by study team members. Specimens, test results or pathogen data will be stored at Duke-NUS EID laboratory in a stand-alone PC whereby access is password protected.

Determination of sample size: To detect an effect size of 0.8 SD in mean viremia level on log(10) scale between first and second with the same vaccine - first dose of will be analysed against those who received YF17D after JE-YF17D and vice versa - a sample size of 25 per group will provide 80% power at 5% two-sided type 1 error rate. To allow for 10% early dropouts, a total sample size of 28 per group for Arm B1 and B2 is targeted. For Arm B3, which is the control group, a sample size of 14 will be used.

Statistical and Analytical plan: Distributional diagnostic plots will be used to examine the shape of distributions of T-cell counts immediately before the challenge vaccination and the subsequent vaccine viremia. Parametric (t-test) or non-parametric (Mann-Whitney U) procedures will be used to assess the differences in T-cell counts and the various measured variables such as viremia, antibody titres and cytokine levels. Pearson's or Spearman's correlation coefficients will be used, as appropriate, to examine the association between T-cell levels and these variables.

The frequency of symptoms reported and the type of symptoms will first be analysed by determining the median and the interquartile ranges. We will then analyse how the first vaccination impact the symptomatic outcome of the challenge vaccination using 2x2 tables and chi square analysis or Fisher's exact test, whichever is appropriate. Parametric (t-test) or non-parametric (Mann-Whitney U) procedures will be used to assess the differences in pre-challenge virus-specific T-cell counts among those with symptoms and those without.

Tests which will be performed on the blood samples: DENV IgG ELISA, FBC, Liver Panel, Viremia (NS5 PCR), cytokines, Anti-NS1 Ab, T-cell studies (AIM, ELLISPOT, ELLA), BCR sequencing, PRNT and gene expression.