CNN  — 

It takes just a tiny virus-laden droplet – about the width of a human blood cell – to infect someone with Covid-19.

That’s just one of the findings from research that deliberately infected healthy volunteers with the SARS-CoV-2 virus. The findings were published Thursday in the journal Nature Medicine.

Challenge studies can be controversial because they involve intentionally giving someone a virus or other pathogen in order to study its effects on the human body. Even with safeguards in place, there’s an element of risk, particularly when studying a new virus.

But they are also hugely valuable for understanding the course of an infection.

“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,” said lead study author Dr. Christopher Chiu, an infectious disease physician and immunologist at Imperial College London.

Volunteers were carefully screened

The study began in March 2021. The 36 volunteers were between the ages of 18 and 30. They were allowed to participate only if they didn’t have any risk factors for severe Covid-19, such as being overweight, having reduced kidney or liver function, or having any heart, lung or blood problems. They also signed an extensive informed consent form to participate.

To further minimize the risks, researchers conducted the study in phases. The first 10 infected volunteers got the antiviral drug remdesivir to reduce their chances of progressing to severe disease. Researchers also had monoclonal antibodies at the ready in case anyone took a turn for the worse. Ultimately, the remdesivir proved unnecessary, and researchers never had to give anyone antibodies.

The volunteers got a tiny drop of fluid containing the originally detected strain of the virus through a long, thin tube inserted into their nose.

They were medically monitored 24 hours a day and stayed for two weeks in rooms at London’s Royal Free Hospital that had special air flow to keep the virus from escaping.

Half were infected

A total of 18 participants became infected, two of whom never developed symptoms. Among the people who got sick, their illnesses were mild. They had stuffy noses, congestion, sneezing and sore throats.

Most of the study participants who caught Covid-19 – 83% – lost their sense of smell, at least to a degree. Nine couldn’t smell at all.

This now-well-known symptom got better for most people, but six months after the study ended, there’s one person whose sense of smell isn’t back to normal but is improving.