Research shows patients hospitalized with influenza or COVID-19 face an increased risk of long-term health problems and death. Since the COVID-19 pandemic began, numerous studies have detailed the virus's ability to attack multiple organ systems, potentially leading to a range of long-lasting and often disabling health problems known as Long-COVID. Now, new research from Washington University School of Medicine in St. Louis and the Veterans Affairs St. Louis Healthcare System shows that,People hospitalized with seasonal flu can also experience long-term negative health effects, especially on the lungs and respiratory tract.

A comparative study of COVID-19 and seasonal influenza

The new study comparing COVID-19 and seasonal influenza viruses also shows that hospitalized patients have an increased risk of death, readmission and health problems in many organ systems within 18 months of being infected with COVID-19 or seasonal influenza viruses. Additionally, the highest risk is on or after 30 days after initial infection.

Key findings and implications

"This study shows that mortality and health losses after hospitalization are high among patients with COVID-19 or seasonal influenza," said study senior author Ziyad Al-Aly, MD, a clinical epidemiologist at the University of Washington. "It's important to note that health risks are higher during the first 30 days after infection. Many people think they are protected from getting COVID-19 or the flu again after leaving hospital. That may be true for some people. But our research shows that both viruses can cause long-term illness."

The findings were published in The Lancet Infectious Diseases on December 14. The statistical analysis spanned up to 18 months after infection and included a comparative assessment of the risk of death, hospital admission and 94 adverse health outcomes involving the body's major organ systems.

Novel research methods

"Looking back at past studies on COVID-19 versus influenza, which have focused primarily on short-term and narrowly defined health outcomes, our new approach compares the long-term health effects of multiple illnesses," Al-Aly said. "Five years ago, I would not have thought of the possibility of studying 'long-term influenza.' An important lesson we learned from SARS-CoV-2 is that infections that were initially thought to cause only transient illness can also lead to chronic illness. This revelation prompted us to study the long-term outcomes of COVID-19 versus influenza."

"We wanted to know if and to what extent patients with influenza also experience long-term health effects," Al-Aly said. The big answer is that both COVID-19 and influenza cause long-term health problems, and the most important "aha moment" was realizing that the long-term health toll was more severe than what these patients endured in the early stages of infection. Long-COVID is a much bigger health problem than COVID, and long-COVID is a much bigger health problem than the flu."

Health risk differences between COVID-19 and seasonal flu

However, the overall risk and incidence of death, hospitalization and health impairment of many organ systems are much higher in COVID-19 patients than in patients with seasonal influenza, Al-Aly said. "The one notable exception is that influenza poses a greater risk to the pulmonary system than COVID-19," he said. "This tells us that influenza is really more of a respiratory virus, as we have thought for the past 100 years. COVID-19, by contrast, is more aggressive and indiscriminate in that it can attack the pulmonary system, but it can attack any organ system and is more likely to cause fatal or severe illness involving the heart, brain, kidneys and other organs."

Data sources and patient demographics

The researchers analyzed de-identified medical records from a database maintained by the U.S. Department of Veterans Affairs, the largest integrated health care delivery system in the United States. They evaluated information on 81,280 patients hospitalized with COVID-19 from March 1, 2020, to June 30, 2022, and 10,985 patients hospitalized with seasonal influenza from October 1, 2015, to February 28, 2019. Patients represented a variety of ages, races, and genders.

For both viruses, a patient's vaccination status did not affect the results. Patients in the COVID-19 cohort were hospitalized during the pre-Delta, Delta, and Omicron phases.

Research findings on COVID-19 and seasonal influenza

Throughout the 18-month study period, COVID-19 patients had a 50% higher risk of death than seasonal flu patients. This equates to about eight more deaths per 100 people in the COVID-19 group than among flu patients.

While the risk of health loss from the COVID-19 virus is higher than that from seasonal influenza, infection with both viruses carries serious risks of disability and illness. The researchers found that across all organ systems, COVID-19 was associated with a 68% increased risk of health conditions (64 of 94 adverse health outcomes), while influenza was associated with a 6% increased risk of health conditions (6 of 94 adverse health outcomes) -- primarily respiratory.

Additionally, patients with COVID-19 are at increased risk of readmission and intensive care unit (ICU) admission within 18 months. For every 100 people, 20 more people were hospitalized in the COVID-19 group than in the flu group, and 9 more people were admitted to the intensive care unit than in the flu group.

The importance of vaccination

"Our findings highlight the continued need to reduce the risk of hospitalization from both viruses to reduce the overall burden of health losses in the population," Al-Aly said. "For both COVID-19 and seasonal influenza, vaccination helps prevent severe illness and reduces the risk of hospitalization and death. Optimizing vaccination rates must continue to be a priority for governments and health systems everywhere. This is particularly important for vulnerable groups such as the elderly and the immunocompromised."

In both COVID-19 and influenza, more than half of the deaths and disabilities occur in the months after infection, rather than in the first 30 days, which is known as the acute phase.

"The idea that COVID-19 or influenza are just acute illnesses ignores their long-term impact on human health," Al-Aly said. "Before the pandemic, we tended to downplay most viral infections as unimportant: 'You're going to get sick and be fine in a few days.' But what we found is that's not everyone's experience. Some people end up with serious long-term health problems. We need to wake up to this reality and stop downplaying viral infections, which are a major cause of chronic disease."

Reference: "Long-term prognosis after hospitalization for COVID-19 and seasonal influenza: a cohort study" by YanXie, Taeyoung Choi, and Ziyad Al-Aly, published on December 14, 2023, The Lancet Infectious Diseases.

DOI:10.1016/S1473-3099(23)00684-9

Compiled source: ScitechDaily