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Explained: What are human challenge trials, which UK will conduct for Covid?

While human challenge trials (HCTs) have helped give important information about several diseases, some have been surrounded by controversy and questions about ethics.  

Written by Mehr Gill , Edited by Explained Desk | New Delhi |
Updated: February 24, 2021 12:59:00 pm
Covid 19 vaccine, human challenge trials, Covid HCTs, what are human challenge trials, Covid 19 clinical trials, express explained, indian expressAn NHS employee looks over the vaccination bays at the Elland Road mass vaccination centre in Leeds, England. (Photo: AP)

The UK is set to conduct the first COVID-19 human challenge trials (HCT) within a month from now. The UK government is spending £33.6 million for the trials, being conducted jointly by the government’s Vaccines Taskforce, Imperial College London, the Royal Free London NHS Foundation Trust and the clinical company called hVIVO.

The trial was first announced in October 2020.

What will happen through this trial?

In this study, over 90 healthy volunteers between the ages of 18-30 will be deliberately exposed to small amounts of the virus in controlled settings, in order to test treatments and vaccines. In this way, the participants are “challenged” by the virus. This is not the same as standard vaccine clinical trials, where the vaccine is given to participants who have acquired the infection naturally.

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Researchers are looking for healthy young adults since they are at relatively low risk of facing complications from COVID-19. Importantly, the researchers will use the strain of the virus that has been circulating in the UK since March 2020 and is known to be of low risk in healthy young adults.

One of the key things that researchers want to determine through the challenge is to identify the smallest amount of virus required to infect a person. Researchers are also hopeful that the results of the initial study will help doctors understand how the immune system reacts to SARS-CoV-2, and to identify factors that influence how the virus is transmitted, including how an infected person transmits the virus into the environment.

While we know that infection from the virus triggers the body’s immune response, which prompts neutralising antibodies to fight off the disease, there are still several unknowns about the virus and how vaccines act against it. For instance, there is little clarity about why some people have no symptoms at all and while others develop mild or severe illnesses.

One of the biggest unknowns about COVID-19 is for how long natural immunity against it lasts. While there are some estimates that suggest that immunity may last for a few months, there is no consensus in the scientific community yet. Last week, France’s health agency announced that it will give only one shot of the vaccine to people who have already had COVID-19, assuming that immunity from natural infections lasts for at least 3-6 months.

Researchers also hope to find out how different vaccines work against the virus, considering that the virus is evolving. A new variant called B.1.525, which appears to be similar to the variant found in South Africa, was identified in the UK this week.

It cannot be said for certain if vaccines significantly impact transmission of the virus and how effective they will be against the new variants. Further, the duration of vaccine-induced immunity also needs to be determined.

What is the purpose of such a trial? 

While HCTs are not a necessary part of vaccine development, some developers request that such a trial be conducted with humans rather than animals. This is because not all conclusions drawn from studying animal models of diseases can be accurately applied to humans.

HCTs are able to give more precise information about the disease and its effects on humans, and also give results about the efficacy of vaccines on infected humans.

Even so, there are certain limitations of HCTs, since they cannot be conducted for diseases that have a high case fatality rate or for diseases for which no course of treatment is available.

The case for and against HCTs 

Human challenge trials have been conducted over hundreds of years and have contributed towards vaccine and drug development. For instance, in the 1900s, HCTs conducted for yellow fever by physician Walter Reed helped to prove that the disease was transmitted by mosquitoes.

A paper published in 2019 in the journal Trials notes that over the last few decades, HCTs for malaria have enrolled over 2,000 participants with no serious adverse events or hospitalisations.

Yet, there is a case against conducting such trials, since some trials from the past will not meet the ethical guidelines of today. While HCTs have helped give important information about diseases such as cholera, dengue, influenza and typhoid, some trials have been surrounded with controversy.

One such trial was conducted in Guatemala in the middle of the 20th century, when participants were exposed to STDs with the goal of finding a suitable prophylaxis (prevention) method for them. Between 1946 and 1948, American researchers deliberately infected vulnerable groups that included sex workers, prisoners, soldiers, mentally disabled and institutionalised people with STDs such as syphilis, gonorrhoea, and chancroid without their consent and knowledge.

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What makes HCTs “ethically complex” for emerging infectious diseases (which COVID-19 is) is the fact that not everything is known about them, and therefore, there is a significant risk of complications.

However, those who support HCTs argue that the potential benefits of such trials outweigh risks associated with them.

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