CHIS, also known as human challenge studies, have been conducted for hundreds of years. One early example was the yellow fever study in the early 1900s to prove that mosquitoes transmit the virus. In these studies, healthy volunteers are purposefully exposed to a disease-causing microbe in a highly controlled and monitored environment. In order to research less dangerous diseases that have effective medications for therapy, a less virulent strain of the bacterium is usually utilised. The majority of the time, they are conducted on a small group of willing participants to better understand the many components of infection and disease, and occasionally to hasten the development of a medical intervention. These investigations are only started as part of vaccine development when safety and immunological responses of the candidate vaccines are understood thoroughly.
Importantly, human challenge studies are conducted to help choose the best candidate for testing in a typical phase-3 clinical trial rather than as a substitute for phase-3 efficacy trials. Less volunteers are frequently needed for phase-3 clinical trials that come after human challenge studies, hastening the development process. Thousands of people have participated in CHIS research over the past 50 years to hasten the development of typhoid and cholera vaccines. 36 volunteers were utilised by Imperial College London to examine various aspects of the SARS-CoV-2 infection during the COVID-19 epidemic. The World Health Organisation approved employing CHIS for quickening the development of the COVID-19 vaccine in 2020.