Almost 90 per cent of Covid-19 patients don't have any of the three main symptoms of the disease on the day they are tested, scientists have revealed.
Researchers analysed data from 36,000 volunteers who were swabbed for the virus between the end of April, a month after lockdown began, and end of June.
But just 16 of 115 people who tested positive for Covid-19 had a fever, cough or had lost their taste or smell — the only symptoms listed by the NHS — on the day their sample was taken.
When fatigue and shortness of breath were added to the 'core' symptoms list, just 23 per cent had the warning signs.
Although 158 people in the survey were suffering from at least one of the three tell-tale symptoms when they were tested, only 10 per cent of those had the virus. This adds to evidence that thousands of people suffering from coronavirus-like symptoms will need to be tested this winter because it is the only way to find out if they have the disease.
The University College London study suggests at least 70 per cent of Covid-19 cases in the UK may be asymptomatic. But others say the actual number is much lower as false-positives in tests could lead to someone wrongly being diagnosed with the virus.
This means the Government's beleaguered testing regime — where people are only tested if they have symptoms of the virus — may be missing thousands of 'silent' transmissions and infections spreading across the country.
Experts have suggested all university students should be swabbed before they head home to enjoy the Christmas break, on the back of the research.
Covid-19 carriers with no symptoms are just as likely to transmit coronavirus as those who feel ill, according to scientists.
Two studies published in September gave further clues about the role played by 'silent' spreaders – who make up around half of those infected.
One study of South Korean Covid-19 cases revealed asymptomatic patients had the same viral load as those with signs of the disease, such a fever, a new and persistent cough or a new loss or change of taste or smell.
Viral load is a measure of how many viral particles someone has circulating in their body and how likely they are to transmit the virus to others by coughing or talking, for example.
It has previously been speculated that those who do not have tell-tale symptoms are infected with a smaller amount of the virus and, therefore, are less likely to spread it.
But some studies say asymptomatic spread accounts for the majority of transmission of the disease, given that people do not self-isolate if they are unaware they have the virus.
The task of uncovering the number of asymptomatic cases is difficult because it relies on screening people at random, which can produce mixed results.
Swiss researchers who analysed 79 studies and estimated that only one in five Covid-19 carriers show no signs of illness at all — far fewer than previously predicted.
It also found asymptomatic people were less likely to pass the virus on to friends and family.
The experts say their study — published in the journal Clinical Epidemiology today — highlights the need to roll-out testing more widely to 'crack-down' on infections.
Professor Irene Peterson, an epidemiologist at University College London who led the study, said their results reveal that 'in the case of university halls, it may be particularly relevant to test all students before they go home for Christmas'.
She added: 'The fact that so many people who tested positive were asymptomatic on the day of a positive test result calls for a change to future testing strategies.
'Future testing programmes should involve frequent testing of a wider group of individuals, not just symptomatic cases, especially in high-risk settings or places where many