WHO spokeswoman Margaret Harris said the UN health agency was planning to issue the guidelines in the next few weeks.

Peter Smith, a professor at the London School of Hygiene and Tropical Medicine and one of the co-authors of a paper published in The Journal of Infectious Diseases in March that kick-started the discussion of HCT for Covid-19, said scientists were hoping to greatly shrink the length of phase 3 clinical trials.

“Phase 3 trials generally take several years. The hope is to reduce this time for Covid-19 vaccines to around six months, but this will depend on the size of the trial and the transmission rate in participants,” he said.

The paper argues that HCT could help achieve the aggressive timeline as they reduce the time and number of volunteers required for testing.

Testing the efficacy of a vaccine is the primary goal of a phase 3 clinical trial, while earlier trials on animals and smaller groups of humans aim to prove the safety and immunogenicity, or immune response, of a potential vaccine.

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Scientists have used human challenge trials in the past, in relation to influenza, malaria, dengue fever, cholera and typhoid fever. Regulatory advice issued by the WHO in 2016 states that this type of trial should only be considered when there are no other effective therapeutics available to treat the disease and prevent death.

Existing ethical guidelines for research related to humans issued by the Council for International Organisations of Medical Sciences and the WHO in 2016 also preclude HCT from diseases with high morbidity rates such as Ebola and anthrax.

Smith said such restrictions should not be applicable to Covid-19, particularly if only young healthy volunteers are recruited.

“Ebola is highly lethal and until recently there were not effective treatments, even now the treatments reduce the high mortality rates, but do not eliminate it,” he said.

“With Covid-19 the risk of serious disease and death is very small in young adults, so it is in a very different ballpark to Ebola.”

The paper co-authored by bioethicist Nir Eyal at Rutgers University in New Brunswick, New Jersey and epidemiologist Marc Lipsitch from Harvard University proposed to choose only volunteers from areas with high transmission rates.

“We actually ask people to take such risks for others’ direct gain every time we ask volunteer firefighters to rush into burning buildings, relatives to donate a live organ to loved ones, healthy volunteers to participate in drug and vaccine toxicity trials with no prospect of improving their health (and some risk of undermining it),” it said.

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The call received a significant push in the past month after several activists, mostly from the United States, started the “1 Day Sooner” campaign to recruit volunteers. Within weeks, nearly 9,000 people from more than 50 countries had signed up.

Josh Morrison, one of the founders, said that he was inspired by Eyal’s paper.

The group’s website says the rate of severe disease and mortality among healthy young people is low, but warns volunteers that they could face serious sickness and even death in outlier cases.

Most of the volunteers who had signed up were young professionals predominantly from English-speaking regions, Morrison said.

No attenuated, or weakened, strain of SARS-CoV-2 is available so far. Morrison said these volunteers are prepared to be infected with the “wild type” or natural strain of the virus.

Dozens of US lawmakers also joined the call to appeal to US regulators to consider accepting findings from human challenge trials as evidence to approve a vaccine candidate.

Global concerns

The discussions have caught the attention of international organisations such as the WHO and foundation Wellcome Trust.

“Vaccines usually take a decade to develop, so if we are to get the Covid-19 vaccine the world so desperately needs within a year, we must work in an unparalleled way. This includes considering the role of human challenge trials,” said Charlie Weller, head of the vaccines programme at Wellcome.

But there were critical questions to address, she said.

“We must carefully consider the conditions under which it would be safe and ethical to establish human challenge trials for Covid-19. For example, we might only decide to do this once we have a highly effective therapeutic in place,” Weller said.

“We also know very little about the longer term health impacts of different age groups of people, so we need to understand who the ‘safest’ group of people might be.”

So far there was not enough knowledge about the long-term impact of Covid-19 to determine if HCT is applicable, she said.

As it took time to design a good HCT model, assessments should be made to see if it could really save time as several candidates had already entered clinical trials, Weller said.

“Can human challenge trials be established and used quickly enough to accelerate vaccine or drug development? … There are no guarantees that these will speed up development but it’s important that we consider [HCT].”

The WHO’s Harris echoed Weller’s concerns.

“With pathogens like this, it is so new in the human population … we are learning every day, and more information is trickling in daily such as what the impacts are,” she said.

The risks should also be fully understood by the volunteers, who should not take them lightly, she said.

“There are people in their 20s or 30s, they don’t end up going to hospital mostly because hospital beds are reserved for people who are severe, or critical. But moderate disease is actually an awful disease, if you speak to people who have gone through it.”

Human challenge trials for vaccine development have always been contentious. A few years ago, an independent ethics committee concluded that HCT for Zika presented a risk to potential volunteers and their sexual partners and that there were other possible study approaches.

Seema Shah, a bioethicist at Northwestern University who was the main author of the ethics panel report on Zika, said it was too early to say if human challenge trials for Covid-19 were ethically justifiable

“Given the uncertainty about risks to participants and whether there will be enough coordination of relevant stakeholders to use the data from a HCT for Covid-19, it is not clear yet whether a HCT is ethically justified,” she said.

She said there were a wide range of standards to meet, including how to manage participants who wanted to withdraw from studies early, how to make sure the studies would not compromise the outbreak response by taking scarce resources and ensuring robust informed consent.

“Nevertheless, there could be considerable value from such a study if everything aligns, and it will take time to prepare, so planning for such studies could commence.”

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In mainland China, scientists are intrigued by the idea, but they said it would be difficult for the public to accept.

“It is difficult to do it in China. Recently there were some articles [online about HCT] and they drew a lot of criticisms,” said Zhu Fengcai, deputy director of the Jiangsu Provincial Centre for Disease Control and Prevention.

Tao Lina, a Shanghai-based vaccine expert, said the risks were too great for researchers.

“Most people would not accept it … it is also hard to pass the ethical requirements,” he said.

Emily Chan Ying-yang, a medical professor at the Chinese University of Hong Kong, said there should be independent committees with the capacity to monitor if promises to volunteers were followed through, such as health care if they got sick.

But most importantly, rigorous safety standards should be ensured and no steps should be skipped, she said.

“We can speed up the process, but you cannot skip any procedure such as animal tests. If we are using human beings as the front line of tests, is that what we can accept?”