In the early days of the pandemic, there was a perception that for the majority of people COVID-19 was a short, relatively mild illness lasting less than a fortnight, with most research focused on the much smaller proportion of patients falling seriously or fatally ill. 

However, in recent months we’ve seen increasing attention paid to people with ‘long COVID’, whose symptoms were not serious enough to land them in hospital yet have persisted for many weeks or months. 

This long-term illness is frustrating and debilitating for those who are affected, with the potential to have a significant impact on wider society. But it hasn’t been clear how many people are suffering from long COVID or who is most at risk.

The latest analysis of data from thousands of users of the COVID Symptom Study app from ZOE, published as a preprint, shows that one in 20 people are likely to suffer from COVID-19 symptoms lasting more than 8 weeks. 

We also found that it’s possible to predict who is likely to develop long COVID based on the very earliest signs of their illness. The more different symptoms a person experienced within the first week of illness, the more likely they were to develop long COVID. Using this together with age, gender and body mass index meant we could accurately predict who is most at risk of developing long-term symptoms.

Counting long COVID

Led by Dr Claire Steves and Prof Tim Spector at King’s College London, this study focused on a subgroup of data from 4,182 COVID Symptom Study app users who had been consistently logging their health and tested positive for COVID-19 through swab PCR testing. We used this subgroup to be sure they definitely had the disease rather than just symptoms which might have skewed the results.

Overall, the team found that while most people with COVID-19 reported being back to normal in 11 days or less, around one in seven (13.3%, 558 users) had symptoms lasting for at least 4 weeks, with around one in 20 (4.5%, 189 users) staying ill for 8 weeks and one in fifty (2.3%, 95 users) suffering for longer than 12 weeks. 

Extrapolating out to the general UK population, which has a different age and gender makeup compared with the COVID Symptom Study app users, the team estimated that around one in seven (14.5%) of people with symptomatic COVID-19 would be ill for at least 4 weeks, one in 20 (5.1%) for 8 weeks and one in 45 (2.2%) for 12 weeks or more.  

Who is most likely to get long COVID?

The researchers discovered that older people are much more likely to get long COVID than younger people, although it does occur across all ages. Long COVID affects around 10% of 18-49 year olds who become unwell with COVID-19, rising to 22% of over 70s. Weight also plays a role, with people developing long COVID having a slightly higher average BMI than those with short COVID.

Although men are more likely to be admitted to hospital with COVID-19, women appear to be slightly more likely to suffer from long COVID than men (14.5% compared with 9.5%), but only in the younger age group.

The researchers also found that people with asthma were more likely to develop long COVID, although there were no clear links to any other underlying health conditions.

Importantly, the more symptoms a person had in the first week, the more likely they were to go on to develop long COVID.  

What does long COVID feel like?

The team found that long COVID symptoms fell into two broad groups. One was dominated by respiratory symptoms such as cough and shortness of breath, as well as fatigue and headaches. The second group was ‘multi-system’, affecting many parts of the body including the brain, gut and heart. 

Long COVID sufferers also reported heart symptoms such as palpitations or fast heartbeat, as well as pins and needles or numbness, and problems concentrating (‘brain fog’).  People who experienced a wide range of symptoms across many body systems were more likely to need a hospital assessment.

People with long COVID were also twice as likely to report that their symptoms had come back again after recovering (relapse) compared with those having short COVID (16% vs 8.4%).

Predicting who will get long COVID

Bringing all the data together, the researchers were able to build a model to predict the likelihood of developing long COVID based on age, gender, BMI and the number and combination of symptoms experienced in the first week of illness. 

Statistical tests showed that this model was able to identify more than two thirds (69%) of people who went on to get Long-Covid (sensitivity), and 73% effective at avoiding false alarms (specificity). The team then tested the model against an independent dataset of 2475 people who reported a positive coronavirus antibody test result with a wide range of symptoms and found that it gave similar results.

This simple model suggests that it should be possible to predict using the app who is more likely to go on to suffer from long COVID, to help target early interventions and research aimed at better understanding and treating this poorly-understood phenomenon.

The scale of the problem

Given that one in twenty people with COVID-19 are likely to be affected by long COVID, the absolute number of people affected by long COVID will only increase as the pandemic wears on, adding up to  potentially hundreds of thousands in the UK and millions worldwide.

Dr Claire Steves, clinical academic and senior author from King’s College London said: “It’s important we use the knowledge we have gained from the first wave in the pandemic to reduce the long-term impact of the second. Thanks to the diligent logging of our contributors so far, this research could already pave the way for preventative and treatment strategies to reduce the long term effects. Using the app daily can help affected people and their doctors better categorise and judge their risks of developing longer more severe disease. We urge everyone to join the effort by downloading and sharing the app and taking just a minute every day to log your health.”

Professor Tim Spector, COVID Symptom Study lead and Professor of Genetic Epidemiology from King’s College London said: “COVID-19 is a mild illness for many, but for over one in 50 people symptoms can persist for longer than 12 weeks. So it’s important that, as well as worrying about excess deaths, we also need to consider those who will be affected by long COVID if we don’t get the pandemic under control soon. Having such large numbers of people affected means specialist services need to be set up urgently with the full financial help for hospitals and GPs. As we wait for a vaccine, it is vital that we all work together to stem the spread of coronavirus via lifestyle changes and more rigorous self isolating with symptoms or positive tests.”

It’s essential that we keep working to understand more about where this disease is spreading and how it is affecting us

We need as many people as possible to download the COVID Symptom Study app and spend just one minute a day logging your health and contributing to the largest COVID-19 research project of its kind in the world - so share the app, not the virus!

Find out more:

• https://www.medrxiv.org/content/10.1101/2020.10.19.20214494v1 
• Long-haulers are redefining COVID-19 - The Atlantic
• ‘Long COVID’: why are some people not recovering? -BBC News
• Long COVID: How to define it and manage it - The BMJ Webinar (for medical professionals)