Influenza Virus-Related Critical Illness

Prevention, Diagnosis, Treatment

Eric J. Chow; Joshua D. Doyle; Timothy M. Uyeki


Crit Care. 2019;23(214) 

In This Article

Risk Factors

Influenza vaccination is the primary method for preventing influenza and reducing the risk of severe outcomes. In the U.S., the Advisory Committee on Immunization Practices (ACIP) recommends annual influenza vaccination for all persons aged 6 months and older and prioritizes those at higher risk for influenza complications.[10] High-risk groups include adults aged > 65 years,[11,12] children aged < 5 years (particularly those aged < 2 years),[13,14] pregnant women (up to 2 weeks post-partum),[15–18] persons with certain chronic medical conditions, Native Americans/Alaska Natives,1 and residents of nursing homes and other long-term care facilities (Table 2). Studies have specifically highlighted that those with chronic pulmonary, cardiovascular, renal, hepatic, neurologic, hematologic or metabolic disorders, immunocompromised persons, children and adolescents receiving aspirin- or salicylate-containing medications and who might be at risk for experiencing Reye syndrome with influenza virus infection, and those who are extremely obese (BMI > 40) are at increased risk for influenza-related complications.[10,19–23]

Many studies evaluated risk factors for severe influenza during the 2009 H1N1 influenza pandemic. Adult ICU patients with influenza A(H1N1)pdm09 virus infection were primarily non-elderly, were obese,[24–28] and had higher odds of death, invasive mechanical ventilation, acute respiratory distress syndrome (ARDS), septic shock, and multi-lobar pneumonia when compared with seasonal influenza patients.[24,29] In children, independent risk factors for influenza A(H1N1)pdm09-related mortality included chronic neurologic condition or immune compromise, acute myocarditis or encephalitis, and early presumed MRSA co-infection of the lung.[30] Female gender was also identified as a risk factor; however, there was no gender difference in overall mortality. Bacterial co-infection was identified in approximately one third of fatal influenza A(H1N1)pdm09 cases in the largest autopsy case series.[31] Bacterial co-infections in the inter-pandemic period are also common in critically ill influenza patients.[32] One study identified past or current tobacco use as a risk factor associated with ICU admission.[33] A recent multicenter cohort study reported that mortality was higher in immunosuppressed patients with influenza A(H1N1)pdm09 than in immunocompetent patients.[34] Severity of influenza seasons varies from year-to-year based on the predominant influenza viruses, and between seasonal and pandemic influenza.[35,36] One study reported that patients with influenza A(H1N1)pdm09 had higher odds of severe disease than patients with either influenza A(H3N2) or influenza B virus infections.[37] However, influenza B virus infection has been shown to increase the odds of in-hospital mortality in children compared with influenza A virus infection.[38]

1These risk factors are included in the U.S. CDC's Advisory Committee on Immunization Practices recommendations for influenza vaccination. This may also apply to indigenous people from other countries, including indigenous Australians and First Nations people.