A<unk> , also called a controlled<unk>  infection trial, is a type of clinical trial for a vaccine or other pharmaceutical involving the intentional exposure of the test subject to the condition tested.<unk> es may be ethically controversial because they involve exposing test subjects to dangers beyond those posed by potential side effects of the substance being tested. During the 21st century, the number of<unk> es has been increasing. A<unk>  study to test promising vaccines for prevention of COVID-19 disease was under consideration during 2020 by several vaccine developers, including the World Health Organization (WHO), and was approved in the UK in 2021. Over the 20th and 21st centuries, vaccines for some 15 major pathogens have been fast-tracked in<unk> es – involving about 30,000 participants who had no serious adverse effects – while contributing toward vaccine development to prevent cholera, typhoid, seasonal flu, and other infections. According to medical ethicists, methods of conducting clinical trials by<unk><unk>  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.
Human challenge study
Platform trial
Analysis of clinical trials
In silico clinical trials
Mixing study
Interphone study
Subgroup analysis
Clinical trial