Covid-19 caused a higher long-term risk of death and adverse health outcomes in nearly every organ system.

The findings by the team from the Washington University school of medicine in St Louis and the Veterans Affairs St Louis Health Care System were published in the peer-reviewed journal The Lancet Infectious Diseases on Thursday.

“This observation emphasises the need for provision of post-acute care following hospital admission for either Covid-19 or seasonal influenza, and that this need is likely to be greater for Covid-19,” the team wrote in the paper.

Flu and Covid-19 are contagious respiratory illnesses caused by different viruses. Research on the pandemic has shown that the coronavirus, in addition to acute illness, could cause lingering problems – including fatigue, shortness of breath and brain fog – collectively known as long Covid.

In the study, the scientists compared the outcomes of more than 81,000 patients treated in hospital in the US for Covid-19 between March 1, 2020 and June 30, 2022 with those for nearly 11,000 patients who were hospitalised with seasonal flu between October 1, 2015 and February 28, 2019.

They found that the Covid-19 group saw eight more deaths per 100 persons compared with the flu group during a follow-up period of a year and a half.

Meanwhile, analyses of organ systems showed that Covid-19 caused a higher risk of disease across all organ systems. The only exception where the risk was higher in flu was the pulmonary system.

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“This tells us the flu is truly more of a respiratory virus, like we have all thought for the past 100 years,” senior author Ziyad Al-Aly, director of the clinical epidemiology centre at Washington University, said.

“By comparison, Covid-19 is more aggressive and indiscriminate in that it can attack the pulmonary system, but it can also strike any organ system and is more likely to cause fatal or severe conditions involving the heart, brain, kidneys and other organs.”

He also highlighted that the health risks were found to be higher after the first 30 days of infection.

“Many people think they are 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.”

Al-Aly said the idea that Covid-19 or flu were just acute illnesses overlooked their larger long-term effects on human health.

“Long Covid is much more of a health problem than Covid, and long flu is much more of a health problem than the flu,” he said.

“Some people are ending up with serious long-term health issues. We need to wake up to this reality and stop trivialising viral infections and understand that they are major drivers of chronic diseases.”

Dr Leung Chi-chiu, a Hong Kong specialist in respiratory medicine, who was not involved in the study, said the results from the US research might not be translatable elsewhere because of different hospital admission criteria in other places and Covid-19 outbreaks involving different variants of the coronavirus.

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He also warned it may not be fair to compare endemic flu with pandemic Covid-19. “Severe Covid-19 and long-term complications appear less common now with the Omicron variant,” he said.

“Higher long-term morbidity and mortality after Covid-19 may be partly attributed to the fact that Covid-19 tends to cause more severe diseases, thus requiring hospitalisation, in those with pre-existing morbidities or risk factors.”