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Landmark study finds one in two hospitalised COVID-19 patients develop a complication

Published: 16 July 2021

A large-scale, NIHR-supported observational study published in The Lancet has found that one in two people hospitalised with COVID-19 during the first phase of the pandemic developed at least one health complication as a result of the viral infection.

The ISARIC/CO-CIN study is the most comprehensive of its kind - and the first to systematically assess a range of in-hospital complications and their associations with age, sex and ethnicity - and their outcomes for the patients.

The researchers assessed more than 70,000 hospitalised adult patients in the UK hospitalised with severe COVID-19 disease between January and August 2020. Of these, half (36,367 of 73,197) developed one or more health complication during their hospitalisation.

The most common of these included:

  • renal complications (24%, 17,752 participants);
  • complex respiratory problems (18%, 13,486 participants);
  • systemic complications (16%, 11,895 participants);
  • cardiovascular problems (12%, 8,973 participants).

But neurological, gastrointestinal and liver complications were also reported. Specifically, the most common medical problems experienced by patients were acute kidney injury, probable acute respiratory distress syndrome, liver injury, anaemia, and cardiac arrhythmia.

Complications by age

The study found high proportions of complications in patients admitted to hospital with COVID-19 across all age groups. People aged older than 60 years were most likely affected - with 54% of 60-69 year olds, 52% of 70-79 year olds, 51% of 80-89 year olds, and 50% of people aged 90 or over developing a complication.

However complications and poor functional outcomes were common, even in younger, previously healthy adults – including in 27% of 19-29 year olds and 37% of 30-39 year olds, 43% of 40-49 year olds, and 49% of 50-59 year olds.

The researchers also found acute complications are associated with reduced ability to self-care at discharge – with 13% of 19-29 year olds, and 17% of 30-39 year olds unable to look after themselves once discharged from hospital.

Complications by gender and ethnicity

Complications were more common in men compared with females, with males aged older than 60 years the most likely group to have at least one complication (women aged under 60 years: 37% [2,814 of 7,689] and men 49% [5,179 of 10,609]; women aged 60 years and over: 48% [11,707 of 24,288] and men 55% [16,579 of 30,416]).

People of white, South Asian, and East Asian ethnicities had similar rates of complications, but rates were highest in Black people (58% [1,433 of 2,480] in Black patients vs 49% [26,431 of 53,780] in white patients).


Following hospitalisation, 27% (13,309 of 50,105) of patients were less able to look after themselves than before COVID-19, and this was more common with older age, being male, and in people who received critical care. The association between having a complication and worse ability for self-care remained irrespective of age, sex, socioeconomic status, and which hospital someone received treatment in. Neurological complications were associated with the biggest impact on ability for self-care.

Dr William van’t Hoff, Chief Executive of the NIHR Clinical Research Network said:

“This study has provided important data to support our understanding of complications arising as a result of severe COVID-19 - including identification of groups most at risk of complications from this dreadful disease. The work, performed in the first phase of the pandemic, underlines how serious COVID-19 is and reinforces the importance not only of the treatments now available, but critically, the role of vaccination in reducing infection and lessening its impact. It also emphasises the need for further research into short and long-term health implications for patients hospitalised with the disease.”

Dr Thomas Drake, co-author from the University of Edinburgh, UK, said:

“Our research looked at a wide range of complications, and found that short-term damage to several organs is extremely common in those treated in hospital for COVID-19. These complications were common in all age groups, not just in older people or those with pre-existing health conditions. People who have complications will often need expert care and extra help to recover from their initial hospital admission.

“We are still studying the participants in our study to understand the long-term effects of COVID-19 on their health. The results from these ongoing studies will be particularly useful, as we found many people who survive COVID-19 and develop complications are from economically active age groups.”

Co-author, Prof. Jonathan Van Tam, Deputy Chief Medical Officer for England said:

“The ISARIC/CO-CIN study was set up at very short notice in March 2020. Ever since it has delivered a steady stream of high-quality data to the UK’s SAGE committee and the Department for Health and Social Care on patients hospitalised with SARS-CoV-2 infection. The project has also produced a series of highly important peer-reviewed science papers which have been heavily cited and are of benefit to scientists worldwide. This is another example of how with support from the UK’s NIHR, the NHS has delivered high quality clinical science studies at pace.”


The study was jointly funded by the NIHR and UK Research and Innovation, while delivery of the study was supported by the NIHR Clinical Research Network and 302 UK hospitals.

It was conducted by researchers from University of Edinburgh, University of Liverpool, University of Sheffield, University of Glasgow, University of Oxford, Public Health England, University of Nottingham, Department of Health and Social Care, and Imperial College London.

The study looked at cases between 17 January and 4 August 2020 before vaccines were widely available, and new variants of the virus had not arisen. However, the authors note that their findings remain relevant in dispelling suggestions that COVID-19 presents no risk to younger healthy adults, many of whom remain unvaccinated.

The authors also note some limitations of the study, including that the data does not provide a long-term picture, and that the timings of complications and patients quality of life were not studied. In addition, the complications in the study were predefined and not specific to COVID-19, so may underestimate some areas as these were added later. In addition, as it was inappropriate to subject patients to numerous tests, patients did not undergo additional tests for complications, and the authors say that the true burden of complications is likely to be higher.

Further information

The Lancet: Characterisation of in-hospital complications associated with COVID-19 using the ISARIC WHO Clinical Characterisation Protocol UK: a prospective, multicentre cohort study

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