[2][3][11] The intent of a challenge study is to fast-track the timeline for providing evidence of safety and efficacy of a therapeutic drug or vaccine, especially by compressing (to a few months) the usually lengthy duration of Phase II–III trials (typically, many years).
[2][3][12] Following preliminary proof of safety and efficacy of a candidate drug or vaccine in laboratory animals and healthy humans, controlled "challenge" studies may be implemented to bypass typical Phase III research, providing an accelerated path to regulatory approval of the test compound for widespread prevention against an infectious disease, such as COVID-19.
[2][9] The design of a challenge study involves first, simultaneously testing a vaccine candidate for immunogenicity and safety in laboratory animals and healthy adult volunteers (100 or fewer) – which is usually a sequential process using animals first – and second, rapidly advancing its effective dose into a large-scale Phase II–III trial in low-risk, healthy volunteers (such as young adults), who would then be deliberately infected with the disease being tested against for comparison with a placebo control group.
[8] Following the challenge infection with or without the candidate vaccine, volunteers would be monitored closely in hospitals or clinics managed by physicians treating people with COVID-19 disease and with life-saving resources, if needed.
These studies have enabled scientists to identify a biomarker for protection and discover that various distinct immune responses precede symptom onset, including some that are also present in individuals who do not become infected upon exposure.
[5] In March 2024, funding for a five-year international consortium to develop and run human challenge studies for mucosal (transmission-blocking) Covid vaccines was announced on behalf of the European Union’s Horizon Europe Programme and the Coalition for Epidemic Preparedness Innovations (CEPI).
[28] Called Mucosal Immunity in human Coronavirus Challenge (MusiCC) and led by Imperial College London, trials are planned to take place in the UK, Europe, Singapore, and the United States.