It only takes 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 of 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 to study its effects on the human body. Even with security measures in place, there is an element of risk, particularly when studying a new virus.
But they are also very valuable in understanding the course of an infection.
"There's really no other kind of study where you can do that, because patients typically only come to your attention if they've developed symptoms, so you miss all those earlier days when the infection is brewing," said lead author of the study., Dr. Christopher Chiu, an infectious disease physician and immunologist at Imperial College London.
The volunteers received a small drop of fluid containing the originally detected virus strain through a long, thin tube inserted into the 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 airflow to prevent 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 a stuffy nose, congestion, sneezing and a sore throat.
Most of the study participants who contracted covid-19, 83%, lost their sense of smell, at least to some degree. Nine couldn't smell anything.
This now well-known symptom improved for most people, but six months after the study ended there is one person whose sense of smell has not returned to normal but is improving.
That's concerning because another recent study found that this loss of smell was linked to changes in the brain.
Chiu says the researchers gave the participants cognitive tests to check their short-term memory and reaction time. They're still looking at that data, but he thinks those tests "will be really informative."
None of the study volunteers developed lung involvement in their infections. Chiu believes this is because they were young and healthy and were inoculated with small amounts of the virus.
Other than the loss of smell, no other symptoms persisted.
A closer look at infection as it moves through the body
Under these carefully controlled conditions, the researchers were able to learn a lot about the virus and how it moves through the body:
Small amounts of virus, about 10 microns — the amount in a single drop that someone sneezes or coughs — can make someone sick.
Covid-19 has a very short incubation period. It takes about two days after infection for a person to start shedding the virus.
People shed large amounts of virus before they show symptoms (confirming something epidemiologists had discovered).
On average, young, healthy volunteers in the study shed virus for 6½ days, but some shed virus for 12 days.
Infected people can shed high levels of virus without any symptoms.
About 40 hours after the virus was introduced, it could be detected in the back of the throat.
It took about 58 hours for the virus to show up on the nose swabs, where it eventually grew to much higher levels.
Lateral flow tests, rapid tests at home, work very well to detect when a person is contagious. The study found that these types of tests could diagnose infection before 70% to 80% of viable virus had been generated.
Chiu says her study emphasizes a lot of what we already know about Covid-19 infections, which is why it's so important to cover your mouth and nose when you're sick to help protect others.