A Phase 2 interventional study of AS01 adjuvant (0.5 mL intramuscular) and YF17D (Stamaril, Sanofi-Pasteur) in Immune System Responses and Trained Immunity After AS01 Administration and Healthy Adult, sponsored by Singapore General Hospital. Recruiting at 1 site in Singapore. Open to participants aged 21 Years to 59 Years, including healthy volunteers. Per ClinicalTrials.gov, last updated 2026-06-15.
Sponsored by Singapore General Hospital · Phase 2, Interventional, and Prevention
As people grow older, their immune system - the body's natural defence against diseases - becomes weaker, making them more vulnerable to infections and less responsive to vaccines. This was clearly seen during the COVID-19 pandemic, where older adults were more likely to develop severe illness. Researchers have made an interesting discovery about AS01, an ingredient already used in successful vaccines like the shingles vaccine. They found clues that AS01 might work like a general fitness trainer for the immune system, potentially making it stronger and better at fighting off various types of infections, not just specific ones. To confirm this possibility, we are conducting this research study with adults aged 21-59 to test whether AS01 by itself can boost and train the immune system, how long this boost lasts, and if it actually helps you fight off other infections more effectively.
As people age, the immune system becomes less responsive, increasing susceptibility to infections and reducing vaccine responsiveness. AS01 is a liposome-based adjuvant used in licensed vaccines (e.g., shingles vaccine) that activates innate and adaptive immunity. Emerging evidence suggests AS01 may also induce trained immunity, a form of innate immune reprogramming that could enhance protection against unrelated infections. This study tests whether AS01 given alone can boost and train the immune system in healthy adults, how long these effects last, and whether this translates into better control of a heterologous viral challenge.
This will be a single-center, randomised, single-blind, placebo-controlled experimental medicine study at Singapore General Hospital (N=40; ages 21-59). Participants receive a single intramuscular dose of AS01 (0.5 mL) or saline placebo on Day 0. To model a controlled viral exposure, all participants then receive the licensed live-attenuated yellow fever vaccine (YF17D, Stamaril) either at 1 month (Day 30) or 3 months (Day 90) after AS01/placebo, per randomization. Serial blood sampling measures immune reprogramming, durability, and response to the viral challenge over \~2 or 4 months depending on assignment
Findings may clarify whether AS01 can be used as a standalone immune booster to rapidly enhance broad protection. Information that could be useful for outbreak preparedness, especially before pathogen-specific vaccines are available.
Therefore, (1) Early and durable innate immune changes after AS01 (e.g., gene expression and epigenetic markers in myeloid/innate cells); (2) YF17D viremia (RNAemia) after vaccination as an indicator of heterologous viral control; and (3) T-cell and B-cell responses to YF17D and how they relate to viremia, will be measured and analysed.
AS01 and YF17D are licensed components when used with their indicated vaccines. Common reactions include local injection-site symptoms and short-lived systemic symptoms; rare serious adverse events have been reported with YF17D. Participants are monitored and provided safety guidance and contact pathways throughout the study.
Singapore General Hospital is the lead sponsor of 266 studies on the registry; 57 are open to participants now.
Of its 8 completed or terminated interventional studies of FDA-regulated products, 0 (0%) have results posted.
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Exclusion Criteria:
Risk factor for live-attenuated vaccines, including any confirmed or suspected primary or acquired immunodeficiency based on history and physical examination:
Participants receive AS01 (0.5 mL IM on Day 0) and yellow fever vaccine YF17D on Day 30.
Biological: AS01 adjuvant (0.5 mL intramuscular) · Biological: YF17D (Stamaril, Sanofi-Pasteur)
Participants receive AS01 (0.5 mL IM on Day 0) and yellow fever vaccine YF17D on Day 90.
Biological: AS01 adjuvant (0.5 mL intramuscular) · Biological: YF17D (Stamaril, Sanofi-Pasteur)
Participants receive placebo (0.9% saline, 0.5 mL IM on Day 0) and yellow fever vaccine YF17D on Day 30.
Biological: YF17D (Stamaril, Sanofi-Pasteur) · Other: Placebo (NaCl 09%, 0.5mL)
Participants receive placebo (0.9% saline, 0.5 mL IM on Day 0) and yellow fever vaccine YF17D on Day 90.
Biological: YF17D (Stamaril, Sanofi-Pasteur) · Other: Placebo (NaCl 09%, 0.5mL)
The AS01 suspension contains 50 mcg/micrograms of 3-O-desacyl-4'monophosphoryl lipid A (MPL) and 50 mcg/micrograms of Quillaja Saponaria Molina, fraction 21 (QS21), licensed by GSK, available as a separate vial from the licensed Shingrix vaccine.
Stamaril, the live-attenuated yellow fever vaccine, utilises the YF17D strain. Stamaril is supplied in the form of powder and solvent for suspension for injection in pre-filled syringe, Yellow fever vaccine (live).
Also known as: YF17D, yellow fever vaccine
The placebo will consist of 0.9% saline solution in equivalent volume.
Also known as: Placebo, Saline placebo, Saline injection
Host Immune Response
Explorative evaluation of host immune response profile (cytokines, immune cell populations, gene expression, epigenetic modifications) induced by AS01 versus placebo, using panel of validated multi-omics assay tests, over a 3-month period.
Time frame: Days 0, 7, 14, 28, 56, 84 (3 months)
Viraemia Levels
Measure Yellow Fever viraemia levels via quantitative RT-PCR and calculate area under the curve (AUC) up to 7 days following yellow fever vaccine administration in AS01 versus placebo recipients.
Time frame: 7 days following yellow fever vaccine administration
B Cell Response
Assess B cell responses to yellow fever by measurement of neutralising antibodies titres using plaque reduction neutralization test (PRNT) with sera at day 30 compared with pre-vaccination day 0, following yellow fever vaccine administration in AS01 versus placebo recipients.
Time frame: 30 days
T Cell Response
Assess T cell responses to yellow fever vaccine using virus-specific IFNγ ELISPOT assay, with results expressed as spot-forming units (SFU) per 10\^6 PBMCs at day 14 and day 30 versus pre-vaccination at day 9 in AS01 versus placebo recipients.
Time frame: 30 days
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Singapore General Hospital