England's Covid-19 test and trace system could not reach a quarter of people who had their cases transferred to the system after a positive test for the new coronavirus, the UK's Department of Health said on Thursday.
Of 6,923 people who had their case transferred to the contact-tracing system in its third week of operation, 1,791, or 25.9 percent, could not be reached, with no contact details provided for a further 263 people.
The test and trace system is seen as key to efforts to lift the lockdown in England, as Prime Minister Boris Johnson allows a growing number of businesses to re-open.
"One in four positive people can't be contacted – this is surprising and worrying," said Keith Neal, an epidemiologist at the University of Nottingham.
"There are a number of issues currently with the system – these can only be sorted by members of the public, as it requires individuals to take the appropriate actions to reduce spread."
Since the programme's launch on May 28, it has reached 113,925 close contacts of the 21,105 people who have tested positive for Covid-19.
"In just three weeks, NHS Test and Trace has already reached more than one hundred thousand people who may have otherwise unknowingly spread the virus," said Dido Harding, executive chair of England's test and trace system.
"It will take all of us working together to stop the spread of the virus... as the country starts to open up again," she added.
In its third week of operation, 81.7 percent of identified contacts of positive cases were reached and asked to self-isolate.
That is down from the 90 percent that were reached last week, although those running the programme cautioned against reading too much into comparisons of weekly data.