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Should Volunteers be Intentionally Infected with COVID-19 to Develop a Vaccine?

Scientists around the world have been working at a breakneck pace to bring a COVID-19 vaccine to fruition, sooner rather than later. As of mid-June, more than 160 potential vaccine candidates were under investigation, and at least some experts are hopeful that at least one of these may turn out to be a viable vaccine that will be ready by the end of this year.

But vaccines can take years develop and at a cost millions of dollars. A large portion of that time is spent on evaluating the vaccine in placebo-controlled phase 3 trials, which generally involve thousands of participants who are followed over a period of time. The goal is then to assess if there is a difference in the disease incidence among the people who received the active vaccine and those who received a placebo.

A faster method, albeit a controversial one, is the so-called human challenge study. In this type of study, volunteers are intentionally infected with the pathogen after receiving an experimental vaccination.  The idea is that it will speed up the process in testing the efficacy of a vaccine candidate, rather than waiting for natural exposure to occur. The concept is not entirely a new one, and was first used in the late 18th century to test the smallpox vaccine. It has since been used to test vaccines for other diseases such as cholera, dengue fever, and influenza. The idea was also proposed 4 years ago when researchers were focused on finding a vaccine for the emerging Zika virus. A group of scientists had proposed the idea to the National Institutes of Health and requested permission to develop a human challenge model. However, the proposal was vetoed by an ethics panel in January 2017, in that it would pose too much of a risk to the participants and their partners.

The idea has now resurfaced, and in article published this past March in the Journal of Infectious Diseases, the authors write that this type of study could replace the conventional phase 3 testing of vaccine candidates. By doing so, this would “subtract many months from the licensure process,” and allow an effective vaccine to become available more quickly. They argue that the “net risk” could be acceptable if the participants are healthy young adults who are at a relatively low risk of serious disease and complications. This risk can be further mitigated if they are monitored at frequent intervals during the trial and if infection does occur, they receive the best care that is currently available.

The idea has been embraced by 35 members of the House of Representatives, who sent a letter to the U.S. Food and Drug Administration (FDA) and the Department of Health and Human Services, asking that this testing option be considered, given the urgency for a vaccine. There also doesn’t seem to be a shortage of people willing to be part of this type of trial. A website called was set up to encourage people to volunteer to be in human challenge trials or to advocate on behalf of them. Right now more than 30,000 people have volunteered and advocates have signed on from 140 countries. But thus far, there are no challenge trials being initiated.

COVID-19 Human Challenge Trials

Cohen, J. United States should allow volunteers to be infected with coronavirus to test vaccines, lawmakers argue. Science. April 21, 2020.

Plotkin SA, Caplan A. Extraordinary diseases require extraordinary solutions. Vaccine. 2020;38(24):3987-3988.

Nir Eyal, Marc Lipsitch, Peter G Smith, Human Challenge Studies to Accelerate Coronavirus Vaccine Licensure, The Journal of Infectious Diseases, Volume 221, Issue 11, 1 June 2020, Pages 1752–1756,

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