Comorbidity Risk Factors and Mortality in Patients Hospitalized with Respiratory Syncytial Virus and Influenza A: a Retrospective Observational Cohort Study

Every winter, the air fills with invisible invaders that target our lungs. Among the most familiar of these are influenza viruses, the agents behind the seasonal flu that has long been recognized as a major cause of illness and death worldwide. In recent years, another virus, known as respiratory syncytial virus or RSV, has stepped out of the shadows. Once thought to be a threat primarily to infants, RSV is now understood to cause significant illness in adults as well, creating a heavy burden on hospitals and families. While doctors have long known that older age and existing health problems make these infections more dangerous, the precise comparison between how deadly these two viruses are for hospitalized adults, and exactly which underlying conditions tip the scales toward death, has remained less clear. Understanding these differences is crucial, especially as new vaccines for RSV become available, helping health officials decide who needs protection most urgently.

A team of researchers in Denmark set out to clarify this picture by looking back at five years of hospital records. They focused on adults who were admitted to hospitals in the Central Denmark Region between February 2019 and February 2024 with a confirmed infection of either respiratory syncytial virus or influenza A. To ensure their findings were accurate, they relied on laboratory test results rather than just the doctors' initial guesses or administrative codes. The study included nearly 4,400 patients, split between those with the flu and those with RSV. By tracking these individuals from the moment they were admitted, the researchers could measure not just who died while still in the hospital, but also who passed away within 30, 90, and 180 days of their admission. This approach allowed them to see the full impact of the infection over time, rather than just the immediate crisis.

The data revealed a stark reality: while more people were admitted with influenza A overall, those admitted with respiratory syncytial virus faced a graver prognosis. The patients with RSV were, on average, older, with a median age of 74 compared to 67 for the flu group. They were also more likely to have at least one pre-existing health condition. Most notably, the death rates were consistently higher for the RSV group across every time frame measured. In the hospital itself, 4.2 percent of RSV patients died, compared to 2.2 percent of those with the flu. This gap widened over time; by 90 days after admission, 10 percent of the RSV patients had died, versus 5.5 percent of the flu patients. Even after adjusting for age, sex, and the number of other health issues a patient had, the risk of death remained significantly higher for those infected with respiratory syncytial virus.

The researchers then dug deeper to understand which specific health problems made the outcome worse. They found that certain chronic conditions acted as powerful predictors of death for both groups. Having chronic obstructive pulmonary disease, a condition that damages the airways and makes breathing difficult, increased the risk of death for both flu and RSV patients. Similarly, chronic liver disease was a major risk factor, appearing to be even more dangerous in the RSV group. However, the study also uncovered some surprising distinctions. For patients with respiratory syncytial virus, having a chronic heart condition was an additional risk factor for death, whereas this was not a significant predictor for the flu group in their analysis. Conversely, a history of asthma showed a protective effect, particularly in the RSV group, where patients with asthma were actually less likely to die than those without it. The researchers noted this was unexpected, as lung disease is usually a warning sign, but suggested it might be because asthmatic patients receive more regular medical care or use specific medications that offer some shield.

Interestingly, the study found that diabetes did not increase the risk of death for either group, challenging the assumption that it is a primary driver of severe outcomes in these specific viral infections. The researchers also highlighted that their method was rigorous; unlike previous studies that might have included factors like pneumonia or the need for a ventilator as part of the risk calculation, this team looked only at conditions the patients had before they arrived at the hospital. This distinction is vital because it separates the patient's baseline health from the immediate severity of the infection, providing a clearer view of who is truly vulnerable before the virus even strikes.

The findings paint a clear picture for the future of public health. Respiratory syncytial virus is not just a childhood illness; it is a serious threat to older adults, carrying a higher risk of death than the flu for those who require hospitalization. The study confirms that age and a heavy burden of other health problems are the primary drivers of this risk, but it also points to specific conditions like heart and liver disease that demand extra attention. As vaccines for respiratory syncytial virus become more widely available, these results provide the evidence needed to target those most in need. The data suggests that protecting the oldest and most medically complex patients is not just a matter of preference, but a necessity to reduce the number of deaths caused by these common respiratory viruses.