The 10-day self-isolation period for Covid could be safely halved because people are barely infectious five days after symptoms develop, studies suggest.
On Monday, the NHS contact tracing app was revised so that people no longer need to quarantine if they were in contact with a symptomless case more than 48 hours earlier.
The change was based on data that show asymptomatic Covid cases rarely pass on the virus more than two days before testing positive. Yet the same studies also showed that the chance of transmission is even less likely five days after symptoms emerge.
Data from Oxford University's Pathogen Dynamics Group, the team which advises the Government on the NHS app, showed that 40 per cent of infections occur before symptoms emerge, and 35 per cent within the first and second day of symptom onset.
In contrast, only about two per cent of transmissions are passed on in days five to 10 after symptoms emerge.
Although the team warned that self-isolation and public health intentions may prevent later-stage infections, a review by the University of St Andrews last year found that the highest risk of transmission occurs a few days before and within the first five days after symptom onset because that is when people are most infectious.
"Given most transmission events happen very early on, the isolation period could be much shorter for the cases," said Dr Muge Cevik, a clinical lecturer in infectious diseases and medical virology at St Andrews, and author of the review, which was published in The Lancet Microbe journal.
"Viral load peaks pretty quickly, so people are highly infectious within the first few days.
"Also importantly, many people have non-specific mild symptoms before developing more noticeable ones, like fatigue or myalgia, so that's probably when people are highly infectious too but continue daily activity.
"So, the current self-isolation guidelines , especially given the lack of support provided for sick leave, does not serve for the purpose."
Last September, France dropped its required period of isolation from 14 days to seven, with Olivier Véran, the country's health minister, saying that "contagiousness decreases in a very significant way" after five days.
Germany also lowered its quarantine period to just five days, while the US allows people to end their quarantine after seven days if they test negative.
More than a million people could be currently self-isolating or quarantining in Britain after testing positive or being pinged by the NHS app, placing an unsustainable burden on businesses which are struggling to find enough staff.
Latest data from Oxford University's Pathogen Dynamics Group show that just 15 per cent of people pinged end up testing positive, a figure which fell below five per cent in April, meaning more than 95 per cent of people were needlessly quarantining.
There is also no way of knowing if the people who went on to develop an infection were infected by their contact, or by another means.
Experts have also warned that the vast majority of transmission occurs between household contacts and close friends, so the chance of picking up a case from a stranger or casual acquaintance is tiny.
Imperial College London estimated that the risk of catching an infection at home was about 20 per cent, compared to 5.9 per cent through social contact with friends and family and 1.2 per cent through a casual contact.
The delta variant has approximately doubled that risk, but it drops by about one third for asymptomatic cases, meaning the current chance of getting infected from a symptomless casual contact is about one per cent.
Prof Paul Hunter, professor in medicine at the University of East Anglia, said: "Having to quarantine for a one per cent risk is marginal, in my view – 100 people have to quarantine for every case transmission.
"Given the state that we are at now, the benefits of quarantining for relatively casual contact do not justify the negative impact, in my view. But if you are positive, still self-isolate and if you live with someone who is positive, still quarantine."
The NHS app also showed that the R rate has dropped substantially since peaking at about July 12, meaning those infected are now passing on the infection to fewer people, so the chance of finding a case among the pinged is getting smaller.
From August 16 , people who have been doubly vaccinated will be allowed to ignore pings if they have no symptoms. However, many younger people will not have had their second jab by then and will still need to isolate for the full 10 days.
Shortening the period of isolation is likely to help with compliance. When the isolation period was originally 14 days, King's College London found just 18.2 people were completely adhering to the rules.
However, more recent data from the Office for National Statistics show that 79 per cent asked to self-isolate are now complying.
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