People hospitalised with seasonal influenza also can suffer long-term, negative health effects, especially involving their lungs and airways, much like the effects of long-term COVID-19, according to a study.

The findings, published in The Lancet Infectious Diseases, showed that Long flu, which was ignored for a considerable time, is real and is a health risk.

The new study by Washington University School of Medicine in St. Louis compared the viruses that cause COVID-19 and the flu also revealed that in the 18 months after infection, patients hospitalised for either COVID-19 or seasonal influenza faced an increased risk of death, hospital readmission, and health problems in many organ systems.

Further, the time of highest risk was 30 days or later after the initial infection.

"The study illustrates the high toll of death and loss of health following hospitalisation with either COVID-19 or seasonal influenza," said Ziyad Al-Aly, MD, a clinical epidemiologist at Washington University.

"It's critical to note that the health risks were higher after the first 30 days of infection. Many people think they're over COVID-19 or the flu after being discharged from the hospital. That may be true for some people. But our research shows that both viruses can cause long-haul illness," he added.

Al-Aly, an assistant professor of medicine at Washington University said that a review of past studies on COVID versus the flu focused on a short-term and narrow set of health outcomes.

They found that the overall risk and occurrence of death, hospital admissions, and loss of health in many organ systems are substantially higher among COVID patients than among those who have had seasonal influenza, he said.

"The one notable exception is that the flu poses higher risks to the pulmonary system than COVID-19," Al-Aly said.

The researchers evaluated health information involving 81,280 patients hospitalised for COVID-19 from March 2020 to June 2022, as well as 10,985 patients hospitalised for seasonal influenza from October 2015 to February 2019.

During the overall 18-month study period, patients who had COVID faced a 50 per cent higher risk of death than those with seasonal influenza. This corresponded to about eight more deaths per 100 persons in the COVID group than among those with the flu.

Although COVID showed a greater risk of health loss than seasonal influenza, infection with either virus carried a significant risk of disability and disease.

The researchers found COVID exhibited an increased risk of 68 per cent of health conditions examined across all organ systems (64 of the 94 adverse health outcomes studied), while the flu was associated with an elevated risk of 6 per cent of health conditions (six of the 94) -- mostly in the respiratory system.

Also, over 18 months, COVID-19 patients experienced an increased risk of hospital readmission as well as admission to an intensive care unit (ICU). For every 100 persons in each group, there were 20 more hospital admissions and nine more ICU admissions in COVID-19 than flu.

"For both COVID-19 and seasonal influenza, vaccinations can help prevent severe disease and reduce the risk of hospitalisations and death, "Al-Aly said.