What is a Human Challenge Trial?
A Human Challenge Trial (also known as a Controlled Human Infection Model, or CHIM) is a clinical study in which healthy volunteers are intentionally exposed to a well-characterised pathogen under controlled conditions.
In a CHIM trial involving a vaccine or antiviral, the well-characterised strain of an infectious agent is given to carefully selected, healthy adult volunteers after they have been vaccinated, or before they receive the antiviral. They follow a traditional double-blind design with an active, treatment group and a placebo group. These trials are performed in specialised quarantine units, where the inoculated volunteers are monitored under 24/7 medical supervision.
In a historical context, the concept of challenge studies is not new. The experiments conducted by Louis Pasteur in the 19th century can be seen as a type of challenge study, where chickens were challenged with a weakened bacterium, causing a chicken cholera and immunised from further chicken cholera infection. Human Challenge Trials have been performed in the UK since 1946 when the Medical Research Council established the Common Cold Unit (CCU) (also known as the Common Cold Research Unit (CCRU)) in the United Kingdom. The aim was to undertake laboratory and epidemiological research on common colds with the aim of reducing human and economic costs.
For early-stage biotech companies, this precision is the real value. Instead of waiting for natural infection rates in large field trials, a challenge model creates a defined setting in which efficacy signals can be observed earlier and more clearly.
Challenge Agents
The pathogen used in a challenge study, the “challenge agent”, is collected from circulating strains, and is then developed, characterised, and manufactured under strict quality standards. Genetic sequencing, dose validation, safety data, and clinical oversight are all part of its development. A wide range of pathogens has been studied in challenge models including influenza, RSV, malaria and SARS-CoV-2.
How Do Regulators View Human Challenge Trials?
Both the European Medicines Agency (EMA) as the U.S. Food and Drug Administration (FDA) have accepted Human Challenge Trials as part of development programs. They have mostly been accepted as proof-of-concept studies and dose finding trials. The WHO has published guidance on the use of challenge trials in vaccine development. The guidance also specifies that if they are performed, human challenge trials may be of particular use:
- When there is no appropriate nonclinical model (e.g., when a candidate vaccine is intended to protect against an infectious disease that is confined to humans)
- When there is no known immune correlate of protection
- When vaccine efficacy field trials are not feasible
For start-up biotechs, this means early interaction with regulators, through a Scientific Advice meeting or a pre-IND meeting, is strongly recommended to discuss the implementation of a Human Challenge Trial in a development program.
How Can Start-Ups Use Human Challenge Trials Strategically?
Capital efficiency and decision clarity are everything for early-stage biotech companies. Human challenge trials can play a role in both. They can generate early efficacy signals in smaller cohorts, inform dose selection, validate correlates of protection, and support stronger scientific advice discussions. Importantly, human clinical data often carries more weight in investor and partnering discussions than animal data alone.
When used correctly, challenge studies can de-risk programs before major capital deployment. When used without a clear regulatory and financing strategy, they risk becoming expensive sideprojects.
The Strategic Lens
Human Challenge Trials are powerful tools, but they are not universally applicable. Their implementation in your development plan needs to be carefully assessed. For early-stage biotech companies, the decision to pursue one should sit at the intersection of science, regulation, and capital planning.
Precision is valuable, but only when it fitsthe broader roadmap.
At Aletheia, we view Human Challenge Trials not as isolated experiments, but as strategic instruments within an integrated development strategy, designed to accelerate clarity, strengthen regulatory positioning, and support confident decision-making.
