Back in March, health technology company ZOE launched the COVID Symptom Study - a simple smartphone app allowing people to log their health and any new symptoms on a daily basis. Since then, more than four million people have downloaded the app and there are currently more than a million weekly users across the UK.

In a powerful demonstration of the utility of this large-scale community science project, insights and analysis from the app by our team at King’s College London have now led to Public Health England updating Covid-19 guidance for older people presenting with delirium, potentially saving many lives.

Identifying delirium as a key Covid-19 symptom in older people

As a geriatrician, I’ve seen the impact of coronavirus infection in older patients first hand, and noticed that a number of my patients with Covid-19 were showing signs of delirium - a state of acute confusion. Delirium can occur during any illness, and can be triggered by low oxygen levels or the effects of the underlying illness on the brain. It is generally more common in older people, especially those who are frail or have issues such as mobility problems, poor eyesight, hearing loss or dementia.

To find out more about the connection between delirium and Covid-19, the COVID Symptom Study app asked questions about confusion, disorientation and drowsiness, which are the core signs of delirium. The app also allowed people to log health reports on behalf of elderly relatives, significantly increasing the amount of data available from older people.

In a recent paper published in the journal Age and Ageing, we analysed data from two groups of older people aged 65 or over. The first included 322 patients hospitalised with confirmed Covid-19, while the second consisted of more than 500 older users of the COVID Symptom Study app reporting a positive coronavirus test result.

Delirium emerged as a key symptom of Covid-19 in frail older people, along with fatigue and shortness of breath, compared with fitter people of the same age. Around one third of app users experiencing delirium did not have the ‘classic’ Covid-19 symptoms of cough and fever. In fact, delirium was the only symptom for around one in five of the hospitalised patients.

Be alert to delirium

Based on our findings, Public Health England rapidly updated its guidance for doctors, advising them to test older people presenting with acute confusion for Covid-19. Healthcare workers and carers should also be on the lookout for delirium and be alert to the fact that it could be the only symptom of coronavirus in frail older people. This is particularly important given that some older people may not be able to report other early signs of Covid-19, such as loss of smell.

While we knew anecdotally from an early stage in the pandemic that delirium seemed to be strongly associated with Covid-19, the large-scale data we’ve been able to gather from millions of COVID Symptom Study app users has enabled us to prove the link is sufficiently strong to warrant a change in practice. The updated guidance will enable quicker diagnosis and earlier treatment of Covid-19 in this particularly vulnerable group, helping to save lives and prevent the spread of the virus.

This isn’t the first time that data from the app has led to a change in health policy. Back in May, we showed that loss of smell (anosmia) was a common symptom of early Covid-19 and a stronger predictor of the disease than fever, leading to a change in the UK’s official list of COVID-19 symptoms. We’ve also highlighted other important common symptoms such as skin rashes, identified fatigue and headaches as early warning signs of infection, and classified six different types of Covid-19 based on symptom clusters.

None of this would have been possible without our millions of dedicated users across the UK taking just a minute or two each day to log their health. 

The COVID Symptom Study app is available to download from the Apple App Store and Google Play Store. Research updates and data can be found at

Dr Claire Steves, Senior Clinical Lecturer, King’s College London