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The Health Secretary announced the rule change for England in the House of Commons saying “that around two-thirds of positive cases are no longer infectious by the end of day five”.
The government has been under increased pressure from the private sector and the NHS recently to do something about the high numbers of people absent from their jobs due to being forced to self-isolate.
There have been calls throughout the week to cut the self-isolation period after the US reduced theirs to five days last week.
Under the move, people will be able to take tests on day five and six but, if positive, must stay in isolation until they have had negative tests on two consecutive days.
Watch: COVID-19: Self-isolation period in England cut from seven days to five, Sajid Javid announces
Some scientists have reacted with alarm at the news, Professor Lawrence Young, Virologist and Professor of Molecular Oncology, Warwick Medical School, University of Warwick, said: “This policy is certainly not following the science"
He said: "Reducing the self-isolation period to five days runs the risk of highly infectious people returning to work or school, particularly if people don’t use lateral flow tests as advised on day five and six."
He added: "A recent study from Japan of 21 hospitalised cases with omicron infection who had been vaccinated has shown that peak infectiousness occurs at three to six days after diagnosis or symptom onset but found that no infectious virus was detected after 10 days.
Prof Young said reducing the isolation period could only work with strict enforcement of lateral flow testing, but the current shortages of tests will make that harder.
He added: "It will also be important to stress that anyone ending isolation on day six should continue to be cautious – wear a face-covering in crowded or poorly ventilated spaces and limit contact with vulnerable people.”
The move to five-day isolation, which comes into effect on Monday, will double the number of people still infectious after coming out of quarantine, according to the government's own analysis.
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According to the modelling by the UK Health and Security Agency, 16% of people are still infectious seven days after first testing positive.
This rises to 31% after just five days.
Javid told MPs COVID is "still with us and there are still likely to be difficult weeks ahead”, but pointed to encouraging signs that infections are falling in London and the East of England, although they are rising in other parts of the country."
Matthew Taylor, chief executive of the NHS Confederation welcomed the move saying: "This is a pragmatic move which leaders will welcome if it can mean more health and care workers who are well enough can return to the frontline, providing it does not significantly add to the risk of the virus spreading."
The UK reported 109,133 new cases of COVID on Thursday, down from the highs of around 200,000 at the start of the month.