Covid: Symptoms appear 3-4 days earlier than thought, study deliberately infecting humans finds

Symptoms of Covid-19 could appear three to four days earlier than existing estimates, a controversial study that deliberately infected people with the virus has found.

The human challenge study, run by a partnership including Imperial College London and the UK Government, deliberately exposed 36 healthy adults aged under 30 to Covid-19.

It was undertaken early last year, when no participants were vaccinated and using the original strain of the virus.

Despite its small sample size and the change to variants and vaccination, researchers said the study revealed important findings which could be applied to the current stage of the pandemic.

A student takes a swab for a lateral flow Covid-19 test on the first day of operation of new asymptomatic testing site in the University of Hull's Allam Sport Centre in Hull, northern England on November 30, 2020. Photo by OLI SCARFF/AFP via Getty Images

One finding was the average time from first exposure to the virus to early symptoms was 42 hours, significantly shorter than existing estimates of five to six days.

Researchers also found higher levels of virus in the nose than the throat, which they said underlined the need for people to wear face coverings over both their nose and mouth.

Lateral flow tests, known to be less accurate than PCR, were shown in the study to be a good indicator of whether someone was infectious.

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Participants were given a low dose of the virus, via drops in the nose, and then carefully monitored for two weeks at the Royal Free Hospital in London.

Of the 36 volunteers, 18 became infected. Two were excluded from the results after developing antibodies, and the remaining 16 developed mild to moderate symptoms, including a runny nose, sore throat, fatigue, muscle aches and fever.

None of the participants developed serious symptoms. Researchers said the study had undergone rigorous ethical checks.

Future research will focus on new variants and vaccinated populations, as well as trying to determine why only half of the participants became infected.

Chief investigator Professor Christopher Chiu, from Imperial College’s Institute of Infection, said the results could be applied to current strains.

"While there are differences in transmissibility due to the emergence of variants, such as Delta and Omicron, fundamentally this is the same disease and the same factors will be responsible for protection against it,” he said.

He added: "Our study reveals some very interesting clinical insights, particularly around the short incubation period of the virus, extremely high viral shedding from the nose, as well as the utility of lateral flow tests, with potential implications for public health."

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