Healthy volunteers were injected with the Covid virus at the Royal Free Hospital to aid research into infections.
The Human Challenge Programme was the first in the world to infect 36 healthy, young volunteers with the virus that causes Covid-19 (SARS-CoV-2).
Participants were monitored from their first contact with the virus until the it was "apparently eliminated".
The landmark study, between Imperial College London (ICL), the Vaccine Taskforce and Department of Health and Social Care (DHSC), Hvivo, and the Royal Free London (RFL) NHS Foundation Trust, took place at a specialist unit at the Pond Street hospital.
Scientists say the results show that experimental infection of volunteers is reproducible and resulted in no severe symptoms in the adult participants, laying the groundwork for future studies to test new vaccines and medicines against Covid-19.
The volunteers given an average of about £4,500 in compensation, depending on their expenses.
Dr Sir Michael Jacobs, consultant in infectious diseases at RFL, said: “We have vast experience of safely managing highly transmissible infections at the Royal Free Hospital and we are really pleased to have been able to play our part in this landmark study.
“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.”
Among key clinical insights, researchers found that symptoms start to develop very fast, on average about two days after contact with the virus.
The infection first appears in the throat, the virus then peaks about five days into infection and, at that stage, is significantly more abundant in the nose than the throat.
They found that lateral flow tests (LFTs) are a reliable indicator of whether infectious virus is present and transmissible.
In the trial, healthy male and female volunteers aged 18-30 years, unvaccinated against Covid-19 and with no prior infection with SARS-CoV-2 were given a low dose of the virus from early in the pandemic via drops up the nose.
They were then monitored by clinical staff for two weeks.
Eighteen of the volunteers became infected, 16 of whom went on to develop mild-to-moderate cold-like symptoms, including a stuffy or runny nose, sneezing, and a sore throat.
Some experienced headaches, muscle or joint aches, tiredness and fever. None developed serious symptoms.
Thirteen reported temporarily losing their sense of smell but this returned to normal within 90 days in all but three participants – the remainder continue to show improvement after three months.
Two participants were excluded from the final analysis after developing antibodies between initial screening and inoculation.
There were no changes seen in participants' lungs, or any serious adverse events in any of them.
The participants will be followed for a year to monitor for any potential long-term effects.
Professor Christopher Chiu, professor of Infectious Diseases at ICL and chief investigator on the trial, said: "People in this age group are believed to be major drivers of the pandemic and these studies, which are representative of mild infection, allow detailed investigation of the factors responsible for infection and pandemic spread.
“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.”
Professor Sir Jonathan Van-Tam, the government's deputy chief medical officer, said: “This important study has provided further key data on Covid-19 and how it spreads, which is invaluable in learning more about this novel virus, so we can fine-tune our response.
"Challenge studies could still prove to be important in the future to speed the development of ‘next-generation’ Covid-19 vaccines and antiviral drugs.
“This data underline just how useful a tool lateral flow tests can be to pick up people when infectious and the importance of wearing a face covering in crowded, enclosed spaces.”
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