First COVID-19 human challenge study yields infection clues

Woman with mask down getting nasal swab
Woman with mask down getting nasal swab

vichie81 / iStock

COVID-19 infection has a shorter incubation period than originally thought, and rapid tests performed well at tracking virus levels, a research team based at Imperial College London reported today in a preprint study that describes the results of the first human challenge trial.

In other developments, a number of countries experiencing Omicron surges reported new record daily cases, while others nations that were affected earlier eased some of their measures.

Infection starts in throat, peaks in nose

With the goals of exploring infection dynamics and whether the human challenge model is safe studying vaccines, treatments, and tests, Imperial College researchers conducted the study in young adults ages 18 to 30 who had not been vaccinated. Participants were inoculated with nose drops containing a low dose of the original SARS-CoV-2 virus, then monitored in a controlled setting for 2 weeks.

Eighteen experienced infections, including 16 who experienced symptoms, which were mild-to-moderate. Common symptoms included stuffy or runny nose, sneezing, musculoskeletal pain, headache, fever, and fatigue. Thirteen had loss of taste or smell, which resolved after 3 months in all but three of the participants.

None developed lung symptoms or suffered adverse events. All will be monitored for 12 months to watch for any long-term effects.

One key finding was a short incubation period of 42 hours, which is much less than the current estimate of 5 to 6 days. The infection first appeared in the throat, with levels peaking highest in the nose. Virus levels peaked at about 5 days, but high levels of viable virus were seen up to 9 days after exposure and as late as 12 days for some of the participants.

Reassuring news about rapid tests

Rapid tests correlated well for swabs, including in asymptomatic people. However, they were less able to flag lower levels of virus at the start and end of infection.

Christopher Chiu, MD, PhD, an infectious disease specialist at Imperial College London and the trial's chief investigator, said in an Imperial College London press release, "Even though in the first day or two they may be less sensitive, if you use them correctly and repeatedly, and act on them if they read positive, this will have a major impact on interrupting viral spread."

Researchers said the peak viral levels they saw in the nose hints at a higher risk of shedding from the nose, underscoring the importance of masks that cover both the mouth and the nose. They also said the infection duration they found supports isolation periods in most guidelines.

In the next research steps, the team will explore why some people became infected and others did not. Work is also underway on a challenge study virus using the Delta variant.

And though there are differences in transmissibility with emerging variants, the disease is still the same, as are the factors for protecting against it, Chiu said. "With a newer strain, there might be differences in terms of size of response, but ultimately we expect our study to be fundamentally representative of this kind of infection."

Michael Jacobs, MD, PhD, one of the authors of the study who is a consultant in infectious diseases at Royal Free London, said, the trial has yielded fascinating new insights into COVID-19 infection. "But perhaps its greatest contribution is to open up a new way to study the infection and the immune responses to it in great detail and help test new vaccines and treatments."

Global pace steady, but record cases in some nations

  • Last week global COVID-19 cases held steady, with the Western Pacific, Eastern Mediterranean, and European regions reporting rises, the World Health Organization (WHO) said yesterday in its weekly pandemic situation report. Deaths were up 6% compared to the previous week. So far, 57 countries have reported the BA.2 Omicron subvariant, thought to be more contagious, and weekly BA.2 prevalence has risen 50% over the past 6 weeks in several countries.

  • Countries reporting record daily cases include Japan, where the total topped 90,000 for the first time, and Turkey.

  • Tonga's prime minister ordered a lockdown that starts today, after its first five COVID-19 cases were detected amid the influx of international assistance as the island nation recovers from the recent volcano disaster.

  • France's government announced that it will start easing COVID-19 restrictions such as outdoor masking and allowing more people to attend mass gatherings.

  • South African researchers will launch studies to investigate coinfections with HIV and COVID-19, amid reports that immunocompromised people with infections such as HIV can harbor SARS-CoV-2 for months, a situation that could lead to new variants, according to Reuters.

  • The global total today climbed to 383,803,710 cases, and 5,695,975 people have died from their infections, according to the Johns Hopkins online dashboard.

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