Adults hospitalised with COVID-19 are at significantly higher risk of complications and death than those with influenza, despite being younger and having fewer chronic illnesses, according to a study conducted in Spain.

The finding, being presented at this year's European Congress of Clinical Microbiology & Infectious Diseases (ECCMID) in Lisbon, Portugal from April 23-26, also suggests that COVID-19 is associated with both longer stays in hospital and intensive care, and costs nearly twice as much to treat.

The researchers from the Hospital del Mar in Barcelona, Spain examined medical records of 187 patients -- average age 76 years and 55 per cent male -- admitted to the hospital with seasonal influenza infection between 2017 and 2019.

They also analysed records of 187 COVID-19 patients -- average age 67 years and 49 per cent male -- hospitalised during the first wave of the pandemic between March and May 2020, who all required oxygen therapy at admission.

In both groups, patients were enrolled consecutively until the required sample size was reached. The study compared clinical characteristics, and healthcare resource use outcomes, including length of stay, admission to intensive care, hospital costs, and death.

Influenza patients tended to have more existing chronic illnesses and problems performing activities of daily living than COVID-19 patients but were less likely to be overweight or obese. The analysis found that COVID-19 was associated with a higher risk of infection severity and admission to ICU.

"Our findings suggest COVID-19 is far more lethal than influenza. Despite influenza patients being older and having more comorbid illnesses, COVID-19 patients had consistently worse health outcomes and were considerably more expensive to treat," said study lead author Inmaculada Lopez Montesinos from the Hospital del Mar.

"Even for those people who are lucky enough to survive COVID-19 and make it out of the hospital, they will be forever scarred by the consequences. It is vital that people get fully vaccinated and boosted against both viruses," Montesinos said.

COVID-19 patients were more likely to experience certain complications such as acute kidney injury, blood clots, and moderate to severe acute respiratory distress syndrome, where the lungs cannot provide the body's vital organs with enough oxygen.

On the other hand, influenza patients were more likely to suffer from bacterial pneumonia, according to the researchers. Overall, 29 out of 187 (15 per cent) COVID-19 patients and 10 out of 187 (5 per cent) influenza patients died of any cause within 30-days of hospitalisation, and the death rate after 90 days was even higher, they said.

The authors of the study noted that there were no differences in mortality trends between the three seasonal influenza periods studied. After accounting for potential confounders including age, comorbidities, sex, disease severity, presence of pneumonia, and corticosteroid treatment, the researchers found that COVID-19 patients were more than three times as likely to die within 30 and 90 days of being admitted to hospital than influenza patients.

Further analyses showed that COVID-19 patients spent far longer in hospital compared with influenza patients, the researchers said. The average cost of critical care for COVID-19 patients was almost twice as much as for influenza patients, they said, adding pharmacy treatment and testing costs were also significantly higher in the COVID-19 group.

The authors acknowledge several limitations of their study, including that it was conducted in one tertiary-care hospital in Spain, so the findings might not be generalisable to other populations. They also noted that no genotyping studies were conducted, and although it is highly likely that COVID-19 patients were affected by wild-type B.1, the results may not reflect the current scenario in which multiple SARS-CoV-2 variants are circulating globally.