Explained: How human challenge works
Coronavirus (Covid-19) Vaccine: In the hunt for a virus, over 20,000 people worldwide have volunteered to infect themselves with coronavirus. How frequent are such trials, what is the objective, and what precautions are taken?
Currently there are over a 100 vaccine molecules under various stages of development across the world. (AP/File)
At a time when the whole world is focused on protecting itself from the novel coronavirus SARS-CoV-2, more than 20,000 people from 102 countries have enrolled on a US website to voluntarily infect themselves with the virus. It is the “human challenge” in the quest for a vaccine against the novel coronavirus disease Covid-19 — the objective is to go on to test whether a given vaccine works on any of these infected persons. Lawmakers in the US have been pushing for human challenge trials and the World Health Organization (WHO) has given the go-ahead, subject to a set of preconditions being met.
What is a human challenge?
According to a recently released WHO Working Group for Guidance on Human Challenge Studies in Covid-19, “Controlled human infection studies (or ‘human challenge studies’) involve the deliberate infection of healthy volunteers. Such studies can be particularly valuable for testing vaccines. They can be substantially faster to conduct than vaccine field trials, in part because far fewer participants need to be exposed to experimental vaccines in order to provide (preliminary) estimates of efficacy and safety. Such studies can be used to compare the efficacy of multiple vaccine candidates and thus select the most promising vaccines for larger studies.”
Currently there are over a 100 vaccine molecules under various stages of development across the world. At least two, one by researchers at Oxford University and another Hong Kong-listed biotech firm CanSino Biologics, are already in Phase II clinical trials. Several members of the US Congress have written to the Commissioner of the Food and Drug Administration and Secretary of Health and Human Services, citing the global cost of the pandemic to consider deliberately infecting volunteers with the virus to expedite vaccine trials.
Human challenges expedite trials because a lot of time may be lost waiting for a trial subject to contract the disease naturally from the community. Till such time that happens, whether the vaccine works or not cannot be tested. Then again, if infection does not happen normally, there is little way of finding out whether it is because of the vaccine or whether it is because the person was never exposed at all.
Has such a measure of deliberately infecting individuals been used on previous occasions?
Human challenge trials are routinely done for diseases such as malaria, dengue, influenza and cholera which extract a heavy public health toll but are not otherwise deadly. Back in 1796 Edward Jenner, father of the smallpox vaccine, infected his gardener’s son with the smallpox virus after having used his newly developed vaccine on the eight-year-old. The child did not get smallpox. Later, Jenner used the deliberate infection strategy on 6,000 other people to test the efficacy of his vaccine that eventually became the principal reason why smallpox could be eradicated. In 1976, the US National Institute of Allergy and Infectious Diseases did a challenge study against cholera.
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According to 1daySooner, the US-based vaccine advocacy group which has started registration of volunteers for the Covid human challenge, “The first well-described influenza challenge study was published in 1937 and involved the inhalation of a human influenza virus. Because only a small number of volunteers (20%) developed mild disease, this model was used across studies for many decades. These challenge studies allowed us to understand more about the human immune response to influenza and test preventative and therapeutic measures. In the 1980s and 1990s the University of Virginia School of Medicine conducted challenge studies with influenza A and B which helped speed the development of oseltamivir (Tamiflu)— an antiviral medication that features on the World Health Organization’s List of Essential Medicines. After the 2009 H1N1 pandemic, researchers at the National Institute of Health (NIH) began using human challenge models to investigate and develop countermeasures against influenza A.”
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WHO says that in the last 50 years, challenge studies have been performed safely in tens of thousands of consenting adult volunteers under the oversight of research ethics committees. These studies have recently helped to accelerate the development of vaccines against typhoid and cholera among others.
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How necessary is this programme for deliberate infection?
There is currently no approved treatment against COVID. This means that there are only two ways of stopping the global march of SARS-CoV-2. The first is herd immunity — when a critical mass of people in a given population get infected by the virus, develop some immunity against it and thus stop being the vessels for further transmission of the disease. This involves a lot of death and suffering; there is also an element of uncertainty because nobody knows how long immunity against Covid lasts in a person who has already had it. Some estimates say 60-70% of a population needs to have been infected by the virus for herd immunity to kick in. The other way of stopping it is a vaccine. That is why there is so much work going on across the world on a Covid vaccine. There are estimates that even a vaccine that clears all trials in a breeze could have a waiting period of 12-18 months before it is available at a global scale.
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A volunteer at trials, led by the University of Oxford, on the newly announced vaccine candidate against Covid-19. (Source: University of Oxford via AP)
Is it ethical to infect people deliberately with a virus that has caused so much suffering?
There are no easy answers to this. That is why it is important to choose volunteers with care, with full disclosure given and informed consent sought from them, before going ahead with the actual act of infection. WHO, in its guidance document says: “Challenge studies are nonetheless ethically sensitive and must be carefully designed and conducted in order to minimize harm to volunteers and preserve public trust in research. in particular, investigators must adhere to standard research ethics requirements. Furthermore, research should be conducted to especially high standards where (a) studies involve exposing healthy participants to relatively high risks; (b) studies involve first-in-human interventions (including challenge) or high levels of uncertainty (for example, about infection, disease and sequelae); or (c) public trust in research is particularly crucial, such as during public health emergencies.”
Based on the data available so far, WHO estimates that participation in SARS-CoV-2 challenge studies would be the least risky for young healthy adults. In ages 18-30 years (whether healthy or not), hospitalisation rates for Covid-19 are currently estimated to be around 1% and fatal infection rates around 0.03%
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