NEARLY one in 20 patients treated in hospital for Covid had no symptoms of the disease and a quarter of all patients have no underlying health problems, according to the largest study in the world to date involving patients from Scotland, England and Wales.

Although a cough, fever, and shortness of breath remain the most common signs of the illness in patients requiring hospital care, affecting around seven in every 10 patients, a minority (4.5 per cent) who were diagnosed as a result of tests while in hospital showed no symptoms at all.

Besides respiratory symptoms, researchers found a cluster of flu-like complaints such as muscle or joint pain, fatigue, and headaches were present in around a tenth of cases, with other patients afflicted by gastrointestinal problems including abdominal pain, vomiting, and diarrhoea.

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These were found in 29% of patients in conjunction with respiratory symptoms, but in 4% of patients diarrhoea, vomiting and abdominal pain were the only symptoms of Covid.

On average, patients had first begun experiencing Covid symptoms four days prior to hospital admission.

The study does not mention anosmia - a loss or change in sense of smell - which was added to the UK's list of Covid symptoms this week.

The authors, who include researchers from Edinburgh and Glasgow, write: "The current case definition of cough and fever, if strictly applied, would miss 7% of our inpatients.

"A smaller proportion, 4% of patients, presented with [gastrointestinal] symptoms only.

"This figure could be an underestimate because these patients fall outside standard criteria for testing."

The findings, published in the British Medical Journal, are based on a peer-reviewed analysis of 20,133 patients admitted to 208 hospitals in Scotland, England and Wales between February 6 and April 19, with follow ups on their condition until May 3.

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Previous studies from China, the US, and South Korea, range in size from 69 to 5700 patients.

The study also found that 23% of Covid patients admitted to hospital had no pre-existing condition, although being male, older, obese and having a health problem such as heart disease, cancer or dementia resulted in a higher risk of dying from Covid.

Increasing age on its own "was a strong predictor of mortality in hospital", with people aged 80 or older with no underlying conditions more than four times as likely to die with the infection compared to a patient in their 50s with no co-morbidities.

The researchers noted that only 1.5% of those admitted to hospital with Covid were under 18, and only 1% of the patients in the BMJ study were under five years old.

"The J shaped age distribution is starkly different to the U shaped age distribution seen in seasonal influenza and the W shaped distribution observed in the 2009 influenza pandemic," they write, adding that is is "not clear" why children are virtually unaffected, although they suggest it could be related to differences in how blood pressure-regulating enzymes function in the lungs of younger people.

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Heart disease was present in 31% of all patients admitted to hospital with Covid; 21% had diabetes without complications; 18% had non-asthma respiratory diseases, such as bronchitis; 16% had chronic kidney disease; and 14.5% had asthma.

However, nearly two thirds of patients were non-smokers who had never smoked.

Nearly one in every five (17%) of all the patients admitted to hospital with Covid required transfer to beds in high-dependency or intensive care units (HDU/ICU).

For patients who only required standard hospital ward care, nearly half (47%) survived their infection, 26% died, and the rest were still receiving treatment at the time the study was completed.

Among the critically ill who required ICU/HDU treatment, 28% were discharged alive, 32% died, and 41% were still in hospital.

Patients with chronic conditions including heart disease, non-asthmatic lung disease, kidney disease, obesity, a neurological disorder such as stroke, dementia, cancer, and liver disease were more likely to die.

The authors write: "Our study identifies sectors of the population that are at greatest risk of a poor outcome, and reports the use of healthcare resources.

"Most patients with covid-19 experience mild disease...17% of patients admitted to hospital required critical care. 

"Factors associated with mortality in hospital were increasing age, male sex, obesity, and major comorbidities."

The results have been shared with the UK Government and World Health Organisation, and are being compared with data from other countries around the world.