A study intentionally exposing young people to COVID-19 discovered exactly how much of the virus is necessary for infection.

Thanks to a new study, we now know how much COVID-19 necessary to infect a person. The study, known as a challenge study, exposed healthy participants to different amounts of COVID-19 and measured how long it took them to catch the virus, and it isn’t much.  According to results, it only takes a small droplet of the virus for people to get infected with the disease.

Researchers conducted their experiment on 36 participants between the ages of 18-29, all of whom hadn’t had previous COVID-19 infections or vaccinations. Participants were screened for co-morbidities, having no extraneous risks to COVID-19. Then they signed a consent form and were exposed to the virus nasally, housed in a containment facility with access to high medical care.

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It was understandably a controversial study, as is the case with most challenge studies, which have some form of risk no matter how controlled the situation. Still, these studies are incredibly valuable, delivering insight that no other studies are capable of. “Really, there’s no other type of study where you can do that, because normally, patients only come to your attention if they have developed symptoms, and so you miss all of those preceding days when the infection is brewing,” lead author Christopher Chu told CNN.

In order to minimize risks, participants were exposed to the virus in stages and provided with the necessary medicine. After exposure, they were monitored 24 hours a day.

Half of the participants got infected with COVID-19, with two of them never developing symptoms. While most infections were mild, most of the infected participants lost their sense of smell, with 9 of them being unable to smell at all. Six months after exposure, one participant still hadn’t fully regained their sense of smell, even though they were slowly improving.

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Despite the risks, researchers learned a lot from the study, like the fact that a droplet in a cough or a sneeze is enough to infect a healthy person. They also determined that it takes two days for people to start shedding the virus, which they then shed for the next 6 to 12 days. But the most interesting aspect of the study was that exactly half of the participants didn’t catch the virus, something that surprised researchers.

The challenge study proved to be a success, inspiring researchers to conduct them on different people, including those who were already exposed to the virus or who are vaccinated. As long as they’re not infecting people at risk, say babies or the elderly, the studies should continue to provide key information as the pandemic develops.