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A sign asks people to wear face coverings to curb the spread of COVID-19, at London St Pancras International rail station in London on Dec. 17, 2021.Matt Dunham /The Associated Press

A novel British study that involved infecting volunteers with the virus that causes COVID-19 has provided insights into why the virus spreads so quickly and could help speed up the development of future vaccines.

Researchers at Imperial College London infected 36 male and female volunteers by dropping a small dose of the virus in their noses. The participants were 18 to 29 years of age, and none had been vaccinated against COVID-19.

The study, known as a human challenge trial, is the first of its kind in the world.

The researchers said none of the participants became seriously ill. Eighteen of the volunteers became infected, and most developed mild to moderate cold-like symptoms, including a runny nose, sneezing and a sore throat. Thirteen lost their sense of smell, but it returned within 90 days for all but three participants, who continued to experience some problems for about three months.

The results showed that it took just 42 hours for the virus to be detected in throat and nose swabs. The amount of virus found in the swabs, a measure of viral load, also rose quickly and peaked after five days.

Those findings challenge the assumption among many public-health officials that the incubation period for COVID-19 is between five and six days. The Imperial researchers also said that even participants who did not exhibit any symptoms recorded very high viral loads within five days.

“They all generated extremely large amounts of virus, which really explains how the pandemic has spread so rapidly,” said Chris Chiu, a professor of infectious diseases at Imperial, who led the study.

“The implication is that a lot of people could be walking around [spreading] the virus who aren’t yet recognizing that they’re symptomatic,” added Wendy Barclay, who heads Imperial’s department of infectious disease. “You may think that you’re healthy and safe to go out and mix with others, but you might not be.”

The study also found that while the virus was first detected in the throat of volunteers, the overall viral load peaked at a much higher level in the nose. That indicates that the virus is more likely to spread via the nose – through sneezing, for example – than via coughing. That finding, the team said, highlights the importance of covering both the nose and mouth with a mask.

The researchers regularly tested participants with lateral flow tests (LFTs) and found that those tests worked well in detecting the virus in early and late stages of infection. “The study provides supportive evidence that LFTs can reliably predict when someone is unlikely to infect others and can come out of isolation,” the researchers said.

Dr. Chiu said the study used a version of the virus that predates the Omicron, Delta and Alpha variants, but researchers plan to conduct further trials on other variants. He also said the scientists will be examining why 16 participants (two withdrew from the study) did not become infected and why some did not develop any symptoms.

“Many of us have devoted our entire careers trying to understand why some people develop symptoms and others don’t and why some people go on to severe life-threatening disease and others don’t,” he said. “This is the kind of study where you can really try and tease that apart, because we know that these participants are all very healthy and that there are none of those other factors which you might find in hospitalized patients, for example, such as a pre-existing heart or lung problem which makes that sort of interpretation much more difficult.”

The success of the study also means researchers will begin developing a trial this year to test future COVID-19 vaccines and antiviral drugs. That could shave months off development times.

Typically, drug makers have to inoculate thousands of volunteers with a vaccine candidate, then wait months for them to become infected naturally in order to test its effectiveness. Human challenge trials expedite the process by infecting a group of volunteers just a couple of weeks after they’ve been vaccinated. These kinds of trials have been used to develop vaccines for the flu, cholera, typhoid and malaria.

The human challenge trial “allows us to develop very rapid, flexible, controlled studies to ask all sorts of questions about infection and immunity,” Dr. Chiu said. “As well as allowing the future development of clinical trials where we can get very early readouts of whether antivirals or vaccines or diagnostic tests work well.”

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