Half of people admitted to hospital in the UK's first coronavirus wave developed at least one complication, according to a study.
It looked at more than 73,000 patients admitted with severe COVID and found half (36,367) developed one or more complications during their stay.
Kidney injury was most common (24.3%), then lung problems (18.4%), followed by heart complications (12.3%).
People who had complications were nearly twice as likely to die and seven times more likely to need intensive care, according to the study - led by Professor Calum Semple from Liverpool University.
Researchers say these complications are different to long COVID symptoms developed by people who've had the virus but did not need hospital care.
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The study found even young, previously healthy people were affected, with 27% of 19 to 29-year-olds and 37% of 30 to 39-year-olds having a complication.
Thirteen percent of 19-29-year-olds and 17% of 30-39-year-olds were so badly affected they were unable to look after themselves when they were discharged.
The figures cover 17 January to 4 August last year, before vaccines were available and the emergence of new variants.
The study's authors say it remains relevant in countering claims that COVID is not a risk to young and previously healthy adults, many of whom are not yet fully jabbed.
"This work contradicts current narratives that COVID-19 is only dangerous in people with existing comorbidities and the elderly," said Professor Semple.
"Disease severity at admission is a predictor of complications even in younger adults, so prevention of complications requires a primary prevention strategy, meaning vaccination," he added.
Patients' complications were assessed at multiple points until discharge.
Of all the 73,197 cases in the study, 56% were men and 81% had an underlying condition.
The average age was 71. Nearly one in three (23,092) died.
Overall, there were complications in 50% of cases, including in the 44% (21,784) who survived.
Dr Thomas Drake, co-author from the University of Edinburgh, said: "Our study shows it is important to consider not just death from COVID-19 but other complications as well.
"This should provide policymakers with data to help them make decisions about tackling the pandemic and planning for the future.
"We are still studying the participants in our study to understand the long-term effects of COVID-19 on their health."
The study is published in The Lancet.
