Two thirds of NHS staff may have been unknowingly infected with coronavirus around the peak of the epidemic, new research suggests.

The University of East Anglia (UEA) and University College London (UCL) surveyed 262 healthcare workers from Barts Health NHS Trust in London in the week of April 17 to 23. 

They found that two thirds had reported anosmia – the loss of taste or smell – before it was considered a symptom of coronavirus.

Of the 73 staff members who had taken a coronavirus test after developing a persistent cough or high temperature, 76 per cent had tested positive for the virus. 

The finding may explain why levels of hospital-acquired infections rose steadily during this period. 

In line with Public Health England guidance at the time of the study, staff who only had anosmia as a symptom would not have been required to isolate or be eligible for testing. 

Papers published by the Scientific Advisory Group for Emergencies (Sage) in June show that during the peak of the epidemic, transmission within hospitals accounted for up to 22 per cent of patients and up to 11 per cent of deaths.

Senior author Professor Carl Philpott, from UEA’s Norwich Medical School, said: “Smell loss as a symptom of coronavirus is particularly important for healthcare professionals because they are at the frontline of pandemic – and at high risk of both contracting and spreading the virus. 

“The really interesting thing that we found was that 168 of the participants – nearly two thirds – said that they had lost their sense of smell or taste at some point between mid-February and mid-April. 

“We also found a strong association between smell loss and the positive coronavirus test results, with those who had lost their sense of smell being almost five times more likely to test positive. This suggests that a large proportion of healthcare workers may have already been infected with coronavirus, with only mild symptoms.”

The team said that although the research was carried out at one NHS trust, they would expect to see similar results across the country. 

“Cases like this most likely went undiagnosed at the time because of a lack of awareness about smell loss as a symptom,” added Prof Philpott.

The study also involved a follow-up survey in May, in which 47 per cent of respondents reported that their sense of smell and taste had completely recovered. A further 42 per cent said they had partially recovered their sense of smell and taste, but just over seven per cent still suffered anosmia. 

The research was published in The Lancet Microbe.

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