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Significant Reduction in Hospitalizations Despite Influenza Vaccine Mismatch

Amesh A. Adalja, MD, FACP, FACEP, FIDSA, October 21, 2016

The 2014-15 influenza season was one notable for a vaccine strain mismatch in the H3N2 component. This mismatch led to a great lack of effectiveness in the prevention of influenza outpatient visits. However, influenza is more than just an outpatient illness, as it causes a wide spectrum of illness that ranges from asymptomatic infection to fulminant pneumonia. A new study published in Clinical Infectious Diseases attempts to determine whether the mismatched 2014-15 vaccine was as ineffective at preventing hospitalizations as it was at preventing outpatient visits.


Vaccine and Hospitalization

In this study, Petrie and colleagues prospectively enrolled adults hospitalized for acute respiratory infections at the University of Michigan Hospital and the Henry Ford Hospital during the 2014-15 flu season. The enrollment period was coordinated to specifically coincide with H3N2 circulation and included 624 participants: 98 with influenza A (H3N2) and 526 influenza PCR test-negative controls. Influenza vaccine had been administered to 67% of enrollees at least 2 weeks prior to illness onset. H3N2 isolates that were pyrosequenced revealed the majority to be the vaccine-mismatched strain (clade 3C.2a). 

These hospitalized H3N2 cases, despite the vaccine mismatch, were significantly less likely to have been vaccinated than controls, implying that the vaccine reduced the severity of illness. Vaccine efficacy for prevention of H3N2 influenza hospitalization was calculated to be 43% overall. In patients less than 50 years of age, efficacy reached 67%. 


The Full Impact of Influenza Vaccine

This study provides important evidence that the standard influenza vaccine, despite its well-characterized flaws, has an effect that is significant and transcends its ability to prevent simple uncomplicated cases of flu. While preventing mild cases of flu has tremendous benefits, which should be conveyed as a major public health message, the ability to prevent severe cases of flu that require hospitalization is highly relevant and desirable. Emergency department crowding, inpatient surge, and ICU bed shortages characterize flu seasons. If vaccination—even in a mismatched season—has the potential to significantly affect the burden of influenza requiring hospitalization, it underscores the importance of increasing vaccination rates with current vaccines while continuing the quest to develop a highly efficacious vaccine that is less subject to mismatch.



Petrie JG, Ohmit SE, Chang CK, et al. Influenza vaccine effectiveness against antigenically drifted influenza higher than expected in hospitalized adults: 2014-2015. Clin Infect Dis 2016;63:1017-1025.