Human Challenge Trials

What if we develop a vaccine for COVID-19 but can’t find enough patients to run a randomized clinical trial? It sounds absurd, but this problem has happened in the past. Ebola was identified in 1976, and candidate vaccines were proven safe and effective in mice and primates in 2004 and 2005, respectively. But no human vaccine was produced [at that time] because it was extremely difficult, bordering on impossible, to trial an Ebola vaccine. The problem? Ebola is so deadly that people take precautionary measures long before a vaccine can be tested.

A few pieces have been written about human challenge trials, clinical trials in which healthy people are infected with a disease in order to see if a treatment or vaccine works, but most of them focus on the ethical issues. I don’t think there are serious ethical issues so writing at The National Interest I focus on why challenge trials are useful statistically and why they may even be necessary.

Even health care workers, however, have a low enough infection rate that you either need many months to determine if there is a significant effect, or you need large populations. In Italy, about 6,000 doctors were infected over two months, out of a population of about 241,000 Italian doctors. This is a monthly infection rate of 1.2 percent. If the vaccine is 50 percent effective, then to detect this within a month, you need a sample size of 7,776 people equally divided between a vaccinated group and a non-vaccinated group. You could run the test in a smaller sample of 1,322 but then the trial would take six months. A more effective vaccine would make detecting an effect easier, but flu vaccines work at 40 to 60 percent effectiveness, so an assumption of 50 percent is not unreasonable.

But will Italian doctors still be getting infected at a rate of 1.2 percent per month when a vaccine becomes available for trial in six months or a year? We hope not. The hope is that social distancing and the use of personal protective equipment will have greatly lowered the infection rate. A low infection rate is great, unless you want to properly test a vaccine.

…The virtue of a challenge trial is that the results would be available very quickly, within a few weeks, and using only a small population. If the vaccine is 50 percent effective, for example, then we would need around 100 volunteers or perhaps even fewer depending on how many people exposed to the virus in laboratory conditions contract the disease.

By advancing a vaccine by many months, a challenge trial could save many thousands of lives and spare the world the huge economic costs of the lockdowns and social distancing that we will be using to combat the virus.

Challenge trials, however, don’t solve all problems. In particular, to limit the risk we would want to restrict the patients in a challenge trial to be young and healthy. But that raises a problem of external validity. We also want the vaccine to be safe and effective in less healthy and elderly people which requires secondary challenge trials or field testing in that population. Nevertheless, as Athey, Kremer, Synder and myself argue in our NYTimes op-ed, the high risk of vaccine failure means that we would like 15-20 vaccine candidates and challenge trials could help us whittle this number down to the best two to three substantially speeding up the vaccine discovery process.

One more point is worth bearing in mind.

[A]n ordinary vaccine trial is not without risk—a vaccine could backfire and make the disease worse—so exposing fifty or so volunteers to the virus in a challenge trial must be balanced against exposing thousands to a potentially dangerous vaccine in an ordinary clinical trial.

Thus, the total risk may be lower with a combination of challenge trials and longer, larger field trials.

Challenge trials have a long history in medicine and their statistical advantages make them powerful and even necessary. As The Guardian notes:

Scientists, however, increasingly agree that such trials should be considered, and the WHO is the latest body to indicate conditional support for the idea.

“There’s this emerging consensus among everyone who has thought about this seriously,” said Prof Nir Eyal, the director of Rutgers University’s Center for Population-Level Bioethics in the US.


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