Although they have similar 90-day death rates, influenza types A and B have unique clinical trajectories, and patients with type A are more likely to receive the antiviral drug oseltamivir (Tamiflu), French researchers report today in CMI Communications.
For the retrospective, multicenter study, the investigators evaluated adults who sought care at one of three tertiary-care hospitals and were diagnosed as having influenza A (234 patients) or B (113) from 2016 to 2018. They also characterized the clinical course by flu type and identified risk factors for poor outcomes across demographic factors, underlying medical conditions, and clinical parameters. The average patient age was 69 years, and 21% were vaccinated against flu.
"While both viruses share overlapping clinical presentations, influenza B circulates less widely due to its exclusive human reservoir and accounts for approximately 23% of annual cases in temperate regions," the researchers wrote. "Studies evaluating the differences in their epidemiology and patient outcomes remain limited, highlighting the need for further studies and targeted interventions."
Clinician perceptions may explain prescribing disparities
Hospitalization rates were high in both groups, with 76.5% of influenza A patients and 70.8% of type B patients requiring admission. The average length of stay was 11 days for influenza A and 12 days for influenza B.
Influenza A and B patients had similar death rates by 90 days (13.4% vs 8.7%, respectively) and proportions of radiologic lung abnormalities (27.8% vs 24.8%), but those with type A were more likely to receive oseltamivir (43.5% vs 27.9% for type B).
"This may reflect clinicians' perception of greater severity in influenza A, as supported by previous studies, given its higher circulation," the researchers said of the oseltamivir prescribing disparities. "However, given the comparable ICU [intensive care unit] admission and mortality rates between influenza A and B, our findings suggest that oseltamivir could also benefit influenza B patients, particularly those with severe disease or risk factors for complications."
Type B disease burden may be greater than believed
Older age (65 years and older) was the strongest risk factor for hospitalization (adjusted odds ratio [aOR], 4.78), while older age (aOR, 3.18), bilateral pulmonary involvement (aOR, 19.4), and receipt of osteltamivir (aOR, 2.43) were risk factors for ICU admission, with a trend toward coinfection with a bacterial pathogen requiring antibiotics (aOR, 2.56).
Despite the fact that influenza B has historically received less attention due to the dominance of influenza A pandemics, recent surveillance suggests that its disease burden may be greater than previously recognized, warranting increased focus in the post-COVID era.
In the short-term, influenza B patients had more favorable outcomes (aOR, 2.52), but chronic respiratory disease and higher scores on the Charlson Comorbidity Index lessened the odds of these outcomes (aOR, 0.34).
"While influenza B patients experienced better short-term (≤5 days) outcomes, overall they experienced more hospitalizations, ICU admissions, and mortality than expected," the study authors wrote. "Indeed, despite the fact that influenza B has historically received less attention due to the dominance of influenza A pandemics, recent surveillance suggests that its disease burden may be greater than previously recognized, warranting increased focus in the post-COVID era."
The findings highlight the need for earlier diagnostic tests and more equitable oseltamivir prescribing, the importance of managing coinfections, and broader vaccination policies tailored to both influenza types, they added.
