A '''human challenge study''', also called a '''challenge trial''' or '''controlled human infection model''' (CHIM), is a type of [[clinical trial]] for a [[vaccine]] or other [[drug|pharmaceutical]] involving the intentional exposure of the test subject to the condition tested.<ref name="lamb">{{Cite journal|last1=Lambkin-Williams|first1=Rob|last2=Noulin|first2=Nicolas|last3=Mann|first3=Alex|last4=Catchpole|first4=Andrew|last5=Gilbert|first5=Anthony S.|date=22 June 2018|title=The human viral challenge model: accelerating the evaluation of respiratory antivirals, vaccines and novel diagnostics|journal=Respiratory Research|volume=19|issue=1|pages=123|doi=10.1186/s12931-018-0784-1|issn=1465-993X|pmc=6013893|pmid=29929556}}</ref><ref name="eyal">{{Cite journal|last1=Eyal|first1=Nir|last2=Lipsitch|first2=Marc|last3=Smith|first3=Peter G.|title=Human challenge studies to accelerate coronavirus vaccine licensure|url= |journal=The Journal of Infectious Diseases|doi=10.1093/infdis/jiaa152|date=31 March 2020|volume=221|issue=11|pages=1752–1756|pmid=32232474|pmc=7184325| name-list-style = vanc }}</ref><ref name="callaway">{{Cite journal|vauthors=Callaway E |date=April 2020|title=Should scientists infect healthy people with the coronavirus to test vaccines?|journal=Nature|volume=580|issue=7801|pages=17|doi=10.1038/d41586-020-00927-3|pmid=32218549|bibcode=2020Natur.580...17C|doi-access=free}}</ref> Human challenge studies may be ethically controversial because they involve exposing test subjects to dangers beyond those posed by potential [[side effect]]s of the substance being tested.<ref name=eyal/><ref name=callaway/>
During the mid 20th and 21st century, the number of human challenge studies has been increasing.<ref name="Adams-Phipps">{{cite journal |last1=BalasingamAdams-Phipps |first1=SJupiter |last2=HorbyToomey |first2=PDanny |last3=Wilder-SmithWięcek |first3=AWitold|last4=Schmit |titlefirst4=TheVirginia|last5=Wilkinson potential|first5=James|last6=Scholl for|first6=Keller|last7=Jamrozik a|first7=Joshua|last8=Roestenberg controlled|first8=Meta|last9=Manheim human|first9=David|title=A infectionSystematic platformReview inof Singapore.Human |journal=SingaporeChallenge MedicalTrials, JournalDesigns, and Safety|datejournal=SeptemberClinical 2014Infectious Diseases|volumedate=5511 |issue=9October |pages=456–612022 |doi=10.116221093/smedj.2014114cid/ciac820 |pmid=25273928|pmc=429393936219704 }}</ref><ref name="cohen16">{{cite journal|url=https://www.science.org/doi/10.1126/science.352.6288.882|title=Studies that intentionally infect people with disease-causing bugs are on the rise|first=Jon|last=Cohen|journal=Science|doi=10.1126/science.aaf5726|date=18 May 2016}}</ref> A challenge study to test promising vaccines for prevention of [[COVID-19]] was under consideration during 2020 by several vaccine developers, including the [[World Health Organization]] (WHO),<ref name="who-chall">{{Cite web|title=Key criteria for the ethical acceptability of COVID-19 human challenge studies|url=https://apps.who.int/iris/bitstream/handle/10665/331976/WHO-2019-nCoV-Ethics_criteria-2020.1-eng.pdf|publisher=World Health Organization|date=6 May 2020|access-date=18 May 2020}}</ref><ref name="cohen20">{{Cite journal|url=https://www.science.org/content/article/speed-coronavirus-vaccine-testing-deliberately-infecting-volunteers-not-so-fast-some|title=Speed coronavirus vaccine testing by deliberately infecting volunteers? Not so fast, some scientists warn|last=Cohen|first=Jon|date=31 March 2020|journal=Science|doi=10.1126/science.abc0006|s2cid=216451224|access-date=19 April 2020}}</ref> and was approved in the UK in 2021.<ref>{{Cite web|title=World's first coronavirus Human Challenge study receives ethics approval in the UK|url=https://www.gov.uk/government/news/worlds-first-coronavirus-human-challenge-study-receives-ethics-approval-in-the-uk|access-date=18 February 2021|website=GOV.UK|language=en}}</ref>
Over the second half of the 20th and the 21st centuries, vaccines for some 15 major pathogens have been fast-tracked in human challenge studies – involving about 30,000 participants who had no serious [[adverse effect]]s – while contributing toward vaccine development to prevent [[cholera]], [[Typhoid fever|typhoid]], [[Flu season|seasonal flu]], and other infections.<ref name="mcmaster">{{Cite web|title=McMaster researcher contributes to WHO guidelines for COVID-19 vaccine testing|url=https://brighterworld.mcmaster.ca/articles/mcmaster-researcher-contributes-to-who-guidelines-for-covid-19-vaccine-testing/|author=Wade Hemsworth|publisher=McMaster University Medical School, Hamilton, Canada|date=13 May 2020|access-date=25 May 2020}}</ref> Since the 1980s, challenge trials which reported about adverse events have had only 0.2% of patients with serious adverse events. <ref name="Adams-Phipps" /> According to [[medical ethics|medical ethicists]], methods of conducting clinical trials by human challenge testing have improved over the 21st century to satisfy ethical, safety, and regulatory requirements, becoming scientifically acceptable and ethically valid as long as participants are well-informed and volunteer freely, and the trials adhere to established rigor for conducting clinical research.<ref name=eyal/><ref name=callaway/><ref name=mcmaster/>
