The news: People who’ve caught covid become infectious far more quickly than previously believed, according to the world’s first “human challenge trial” study in which healthy young volunteers were deliberately infected with the virus. The study, carried out by a team led by researchers at Imperial College London, is the first to watch what happens from the moment someone is infected with SARS-CoV-2.
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The findings: The 36 volunteers, all aged 18-30, were exposed to a low dose of the original SARS-CoV-2 virus in the nose, the equivalent of the amount found in just a single drop of nasal fluid. Half of the participants developed covid symptoms, and became infectious within just two days, with levels of infectious virus peaking at five days. It has previously been estimated that it took about five days from exposure to first symptoms. Participants in the study remained infectious for an average of nine days and still had detectable levels of virus in their nose 12 days after initial exposure.
Almost all of the volunteers lost their sense of smell and experienced cold-like symptoms like a runny nose, and a sore throat. None reported serious symptoms. Some of the patients were also given the antiviral drug remdesivir before they were infected but the trial didn’t pick up any noticeable difference in the severity of the symptoms.
What it all means: The findings come with the caveat that they’ve been derived from a small pool of volunteers, and are published in a pre-print paper that has not yet been peer-reviewed. However, they provide useful insights nonetheless. The fact that people become infectious so quickly and stay infectious for so long suggests that recommended isolation periods should be kept at around ten days. Although the virus was detected in the throat first, it was eventually present at much greater levels in the nose, highlighting the need to wear face masks properly so they cover the nose.
Get tested: The research also supports the regular, widespread use of lateral flow testing. Modeling using the study data found that regular rapid tests can diagnose infection before 70-80% of infectious virus had been generated, meaning that if people tested regularly and isolated when positive, it could significantly cut community transmission. The fact that none of the participants fell severely ill also suggests this challenge trial method could be used to test future variants or drugs in the future.
Dr Sir Michael Jacobs, consultant in infectious diseases at the Royal Free London hospital, where the trial was carried out, said in a statement: “The trial has already provided some fascinating new insights into SARS-CoV2 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.”