The first "controlled investigation" of Covid-19 has been carried out and it has allowed researchers to dispel a widespread myth about the way people become infected.
The human challenge study involved people being deliberately infected with a virus — in this case it was SARS-CoV-2, the virus that causes Covid-19.
The study, published last week in the journal Nature Medicine, found that only the most minuscule amount of the virus is needed to infect a person — as much as a single airborne droplet from a person sneezing, coughing or talking.
It also found that, despite what most people have been told, viral shedding and transmissibility occur at high levels when a person is infected regardless of whether they have severe or mild symptoms.
Researchers from University College London Hospital, Imperial College London and the University of Oxford, among others, were also able to bust another myth, one pushed mostly by anti-maskers.
They found the virus is "present at significantly higher [levels] in the nose than the throat", a finding they said provides "clear evidence that emphasises the critical importance of wearing face coverings over the nose as well as the mouth".
"Deliberate human infection of low-risk volunteers enables the exact longitudinal measurement of viral kinetics, immunological responses, transmission dynamics and duration of infectious shedding after a fixed dose of a well-characterised virus," authors wrote.
"Experimental challenge with human pathogens requires careful ethical scrutiny and regulation but can deliver unparalleled information that may inform clinical policy and refinement of infection control measures."
Those "unparalleled findings" were made possible thanks to 36 volunteers aged between 18-30 years.
The participants were each given a dose of the virus via a small tube into their nose, then housed in a high-containment quarantine unit at the Royal Free London NHS Foundation Trust with 24-hour close medical monitoring and full access to clinical care.
Researchers found that 18 participants — equating to 53 per cent — became infected with a viral load "rising steeply and peaking five days after inoculation".
They said the virus was "first detected in the throat but rose to significantly higher levels in the nose".
"Viable virus was recoverable from the nose up to 10 days after inoculation, on average."
The researchers found that most of those who became infected recovered quickly. There were no serious adverse events and "mild-to-moderate symptoms were reported by 16 [of the 18] infected participants".
The other two who were infected remained asymptomatic.
The researchers also looked at how Covid-19 impacts a person's sense of smell.
They used smell identification tests with help from the University of Pennsylvania and found that 15 of the 18 participants who were infected reported "some degree of smell disturbance".
Nine participants experienced complete smell loss, known as anosmia, but researchers said they "improved noticeably before day 28".
Six months after the study ended, one of the participants has still not regained their full smell.
It comes after several recent groundbreaking studies into the way the virus spreads and how to best treat it.
One study found that Ivermectin, the anti-parasitic drug popularised as a treatment for Covid-19 by podcast host Joe Rogan, is "useless" against the disease.
Researchers with the TOGETHER study, a worldwide project involving a series of clinical trials designed to test the effectiveness of several repurposed drugs to treat Covid-19, conducted a double-blind, randomised, placebo-controlled trial involving 3515 patients at clinics in Brazil.
Patients who had had symptoms for up to seven days and at least one risk factor were randomly assigned to receive ivermectin or a placebo, with 679 people in each group – 2157 received another intervention.
Those who received ivermectin were given a dosage of 400 micrograms per kilogram of body weight, once daily for three days.
"Treatment with ivermectin did not result in a lower incidence of medical admission to a hospital due to progression of Covid-19 or of prolonged emergency department observation among outpatients with an early diagnosis of Covid-19," concluded the paper summarising the findings, published this week in the New England Journal of Medicine.