Researchers Deliberately Infected People With COVID—Here's What They Learned

COVID-19 test.

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Key Takeaways

  • Challenge studies intentionally infect participants under very strict regulations to observe the full lifecycle of a virus.
  • A U.K.-based study purposely infected 36 volunteers with COVID-19.
  • Researchers gained insight into how the virus works in younger, healthy adults.

With great risk comes great reward—at least that’s what researchers were betting on when they began a COVID-19 challenge trial this year.

The study, sponsored by Imperial College London, intentionally infected 36 healthy people between the ages of 18 and 29 with COVID-19. Now initial results of the study are available in preprint, which means they have not yet been peer-reviewed.

Challenge trials are studies where participants are intentionally infected with a virus or other pathogen. Scientists value challenge studies because of the opportunity to study infection from its origin. By doing so, researchers can observe details that might otherwise be missed.

Catherine Gregor, Chief Clinical Trial Officer at Florence Healthcare, shared that challenge studies are not a new idea and have been used throughout medical history.

“This type of study is not new,” Gregor told Verywell via email. “It has been used throughout history to help researchers better understand how different viruses affect the body, going all the way back to the first smallpox experiments. What has changed over time is the rigor and regulatory requirements associated with clinical trials. Challenge studies and first in human studies are much more tightly controlled in today’s environment which in practice makes them safer for participants.”

In this study, droplets of a strain of COVID-19 predating the Alpha variant were administered by droplets through the nose. After exposure, participants were observed in a tightly controlled setting. Those that developed infections were treated with remdesivir immediately.

Insight Into the Virus

Since there are so many ethical concerns with challenge studies, the study was small, with only 36 participants chosen out of 26,000 volunteers. Among those exposed to the virus, only 18 developed COVID-19. Gregor said that this could be the subject of further study to investigate what biological factors may have protected some from infection.

Among those infected, researchers observed that the infection began extremely quickly—within two days of first exposure. Prior estimates of infection times ranged from five to six days from exposure.

After becoming symptomatic, participants saw a spike in symptoms up to the fifth day of infection, at which point, virus counts began to fall. The virus was still detectable by a rapid test up to 12 days after symptom onset. With such a long period of viral presence, longer quarantine times may make sense. (Though, the way the virus works may change depending on the variant.)

Although the virus was detectable by tests for a longer period than expected, researchers noted that lower viral loads at the beginning and end of the infection period were harder to detect. Testing too early could lead to missed infections.

While the virus was initially detected in the throat, despite the sample virus being administered through drops in the nose, it built the strongest colonies of virus in the nose. Scientists say that this indicates that the virus is much more likely to be spread through the nose, making proper mask-wearing essential to curbing virus transmission.

Of the 18 infected participants, only 13 of them lost their sense of smell, which has been considered one of the most reliable predictors of COVID-19 infection. All but three participants regained their sense of smell within three months.

What This Means For You

While controversial, the challenge study gave researchers an interesting look at how the virus affects younger, healthy people. It also further proved that properly worn masks can mitigate the spread of the virus since the viral load was most concentrated in the nose.

Continuing Research

While researchers have released their initial findings, they will continue to follow all the participants in the study for the next 12 months to observe any further developments.

Gregor said that this small study could be a blueprint for expanded research in the future.

“Small sample size is common in early phase research. Pilot studies like this one are looking to determine the most relevant sample size for larger studies by providing information on standard deviations in the outcome data,” she said. “This information can then be used to power the larger study and help determine how many more participants need to be included to make positive assumptions about the general population.”

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.

1 Source
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Killingley B, Mann A, Kalinova M, et al. Safety, tolerability and viral kinetics during SARS-CoV-2 human challenge. Research Square. Preprint posted online February 1, 2022. doi:10.21203/

By Rachel Murphy
Rachel Murphy is a Kansas City, MO, journalist with more than 10 years of experience.