Only half of healthy young Britons exposed to Covid actually get infected, research suggests.
The results come from a first-of-its-kind study, which deliberately infected 34 people aged between 18-30 who had no underlying conditions. Volunteers were compensated £4,565 to have droplets of the Wuhan strain put into their nose.
Imperial College London scientists found just 53 per cent of the group succumbed to an infection — defined as testing positive twice.
Exactly why the remaining volunteers resisted the virus remains a mystery, although experts suspect it is down to differences in their immune responses.
Experts believe cross-protection from other coronavirus infections could explain why some people ward off Covid.
The study — known as a challenge trial — also discovered people develop symptoms within two days of being exposed to the virus. For comparison, public health bosses believed the incubation period was around five days.
However, the findings, yet to be peer-reviewed, are based on healthy young people, who were not vaccinated or previously infected.
The study was conducted in summer 2020, before any vaccines were available. But the Office for National Statistics estimates 98 per cent of Britons are now jabbed or infected.
Researchers insisted the results are still relevant and pave the way for future studies to infect people with the virus.
Professor Christopher Chiu, the study’s lead author, claimed there are differences in transmissibility due to the emergence of variants, such as Delta and Omicron.
But he added: ‘Fundamentally, this is the same disease and the same factors will be responsible for protection against it.
‘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.’
The researchers found it took just 42 hours from being exposed to Covid to test positive from a throat swab (graph, right), compared to 58 hours to test positive on a nose swab (graph, left) based on samples taken from the18 Britons who became infected during the study. Viral load remained lower and peaked earlier in the throat. Higher levels of the virus in the nose suggest people are more likely to shed the virus when breathing from their nose, the researchers said
The researchers estimated that infected people shed Covid for 6.5 days on average, with the first positive test being taken from the throat within an average of 42 hours. The graph shows the length of time people in the study tested positive for from throat (top) and nose (bottom) samples, with the red line representing the median length
Volunteers were all kept isolated for at least 14 days for the study and had access to entertainment, including a TV and games console.
Drops of the original Wuhan SARS-CoV-2 strain were administered up the nose at the lowest possible dose known to cause infection — roughly the equivalent to a single droplet of nasal fluid from an infected person.
Just 18 of the 34 volunteers, who were kept in a specialist unit at Royal Free Hospital in London, became infected (53 per cent).
Investigations into how the other 16 participants avoided infection are taking place.
Most of the infected (89 per cent) developed mild or moderate cold-like symptoms, such as a stuffy or runny nose, sneezing and a sore throat.
Lockdowns, school closures and limiting gatherings only reduced Covid mortality by 0.2%, study finds
The original coronavirus lockdowns had ‘little to no’ effect on pandemic death tolls in the US, UK and Europe, a controversial report suggests.
Economists who carried out a meta-analysis estimated that draconian restrictions first imposed in spring 2020 — including stay-at-home orders, compulsory masks and social distancing — only reduced Covid mortality by 0.2 per cent.
They warned that lockdowns caused ‘enormous economic and social costs’ and concluded they were ‘ill-founded and should be rejected as a pandemic policy instrument’ going forward.
The review, led by a Johns Hopkins University professor, argued that border closures had virtually zero effect on Covid mortality, reducing deaths by just 0.1 per cent.
However, the researchers estimated closing nonessential shops was the most effective intervention, leading to a 10.6 per cent drop in virus fatalities.
Their report, which has not been peer-reviewed, said that this was probably due to shutting pubs and restaurants where alcohol is consumed. School closures were linked to a smaller 4.4 per cent decrease.
The researchers originally identified 18,590 global studies into lockdowns, which they claim had to be whittled down to just 24 to answer their research question.
Critics have accused them of ‘cherry-picking’ studies to suit their narrative and have raised doubts about the biases of its authors, who have been vocal about lockdowns and vaccine mandates on social media.
Most scientists believe that, before the arrival of vaccines and antivirals, lockdowns had a significant effect on cutting transmission and therefore reducing the number of hospital admissions and deaths caused by Covid.
Some of the infected patients also experienced headaches, muscle and joint pain, tiredness and a fever, but none developed serious symptoms.
But immunity, from either vaccines or a prior infection, is now higher among the population than it was when the study took place, reducing the risk of symptoms.
All those who became infected generated ‘very high levels of the virus’ even if they had no symptoms, the researchers also found.
Covid symptoms started within 42 hours of coming into contact with the virus, on average.
The team noted this was ‘significantly shorter’ than current estimates, which say it takes around five days for symptoms to start.
After this point, there was a ‘steep rise’ in the amount of virus in swabs taken from the volunteers’ nose and throat. The viral load peaked five days later, on average.
The team found people shed the virus for 6.5 days after being exposed to the virus, on average.
Yet ‘high levels’ of the SARS-CoV-2 virus were still detected for up to 12 days in some participants.
Traces of the virus were picked up in the throat earlier than the nose — 40 hours compared to 58 hours — but throat swabs had much lower levels of the virus and peaked much earlier.
The researchers said this indicates there is a ‘potentially greater risk’ of the virus being shed from the nose rather than mouth, highlighting the importance of wearing face masks over both nose and mouth.
The team also found that lateral flow tests are a ‘reassuringly reliable indicator’ of whether someone is still able to transmit the virus to other people.
Positive lateral flow tests ‘correlated well’ with positive PCR tests throughout an infection, including among those with no symptoms.
But the tests were less likely to detect the virus at the start and end of an infection.
The team said this supports using the rapid tests, which can ‘reliably predict when someone is unlikely to infect others and can come out of isolation’.
Professor Chiu said: ‘We found that overall, lateral flow tests correlate very well with the presence of infectious virus.
‘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.’
The researchers noted that the small number of volunteers may have affected the findings.
But the results have ‘important implications for public health’, including around mask wearing, isolation periods, the use of lateral flow tests and infecting people to further investigate the virus, the team said.
The researchers are already developing a similar study that will expose volunteers to the Delta variant.
The Department of Health, the Vaccine Taskforce, the Royal Free Hospital in London and clinical trial developer hVIVO also worked on the study.
Professor Chiu added: ‘People in this age group are believed to be major drivers of the pandemic.
‘And these studies, representative of mild infection, allow detailed investigation of the factors responsible for infection and pandemic spread.
Professor Sir Jonathan Van-Tam, England’s departing deputy chief medical officer, said the study ‘provided further key data on Covid and how it spreads, which is invaluable in learning more about this novel virus, so we can fine-tune our response’.
He added: ‘Challenge studies could still prove to be important in the future to speed the development of next-generation Covid vaccines and antiviral drugs.
‘These data underline just how useful a tool lateral flow tests can be to pick up people when infectious ad the importance of wearing a face covering in crowded, enclosed spaces.’