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Greater H1N1 immunity found among younger people in 2013-2014
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Unlike the 2009 pandemic influenza season when influenza A(H1N1) pmd09 was last predominant, the 2013-2014 season’s mortality risk factors shifted to those of a more typical epidemic influenza season, according to data recently published in Infection Control and Hospital Epidemiology.
Most notably, there was reduced risk among children and young adults, suggesting greater immunity to the strain in this population, they wrote.
Nirav S. Shah, MD, MPH, department of medicine at the University of Chicago, and colleagues retrospectively reviewed patients with laboratory-confirmed influenza admitted to ICUs at 33 medical centers involved in the Severe H1N1 Influenza Consortium 2014. The study included 444 adults and 63 children, all admitted to an ICU from September 2013 to April 2014.
At each facility, data from electronic health records included patient race and ethnicity, comorbidities, BMI and vital signs. Using these records, researchers calculated the Sequential Organ Failure Assessment (SOFA) score based on the first 24 hours of ICU data.
The most common presenting symptoms in adults were cough (79.5%), shortness of breath (77.4%), fever (56.4%) and malaise (44.5%); in children, fever was the most common symptom (76.2%), followed by cough (73%), shortness of breath (68.3%) and nasal congestion (60.3%), the researchers wrote.
Among participants, 93% of adults and 81% of children had underlying medical conditions that put them at greater risk for severe disease, the researchers wrote. For example, 48% of adults were obese, and 4.5% of the women enrolled were pregnant.
Although the majority of patients in this study received antiviral therapies, only 26.4% of adults and 56.1% of children received the treatment within 48 hours of symptom onset.
The researchers found that risk factors associated with increased mortality in adult patients during the 2013-2014 influenza season were: a history of chemotherapy for a malignant tumor within the last 6 months (OR = 12.1; 95% CI, 3.9-37), aged older than 65 years (OR = 3.1; 95% CI, 1.4-6.9), male sex (OR = 1.9; 95% CI, 1.1-3.3) and a higher SOFA score (OR = 1.3; 95% CI, 1.2-1.4).
“The age distribution of patients with severe influenza in our study demonstrated a shift to older adults compared with the 2009 pandemic year,” Shah and colleagues wrote. “Given that the genome of the influenza A (H1N1) pdm09 virus circulating in 2013-2014 was similar to the virus circulating during 2009, this age shift may reflect increased population-level immunity among younger people resulting from H1N1 infections or from vaccination during the preceding 4 years.” – by Colleen Owens
Disclosure: The researchers report no relevant financial disclosures.
Perspective
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William Schaffner, MD
This paper describes the clinical impact of influenza during 2013-2014. The dominant strain during that influenza season was the influenza A(H1N1)pmd09 virus that first burst upon the scene in 2009, causing a worldwide pandemic. Back then, it disproportionately affected children and young adults. Influenza A(H1N1)pmd09 continued to circulate in subsequent years and was the dominant strain in 2013-2014. By then, however, its epidemiologic characteristics had changed; now it wreaked its worst effect on the more traditional population of older adults and those with underlying illnesses as described so clearly by Shah and colleagues, thus reinforcing that old adage, “If you’ve seen one flu season … you’ve seen one flu season.” Sadly, we learned that lesson yet again last year. Just as the annual influenza vaccination effort was getting started, it became evident that the H3N2 strain had mutated, rendering that component of the vaccine much less effective. As feared, this newly mutated strain became the dominant virus causing disease.
The vaccine has been reconstituted for this 2015-2016 season and now includes the new H3N2 strain that many of us anticipate will continue to dominate, providing motivation to promote vaccination. The recommendation from the CDC’s Advisory Committee on Immunization Practices (ACIP) really is simple: Everyone older than 6 months of age should be vaccinated — unless they have a medical contraindication, which fortunately is rare. Be assertive — do not let patients leave your office unvaccinated!
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