WASHINGTON — The influenza A/H1N1 pandemic of 2009 to 2010 raised the question of whether some flu strains are inherently more likely to cause severe illness than others.

As a result, many studies have described the clinical course and outcome of the different subtypes of influenza—A/H1N1, A/H3N2 and B. New DoD research identified no appreciable differences, however, in either clinical presentation or disease severity among the subtypes.

The report in Military Medicine cautioned that study parameters limit the applicability of the results to military populations.1

Madigan Army Medical Center-led researchers sought to evaluate differences in disease severity among influenza subtypes in a cohort of healthy, primarily outpatient adult U.S. DoD beneficiaries. To do that, the study team enrolled otherwise healthy adults up to age 65 with influenzalike illness in an observational cohort study based in five U.S. military medical centers from 2009 to 2014. Researchers collected serial nasopharyngeal swabs for determination of etiology and viral-shedding by polymerase chain reaction. At the same time, the presence and severity of symptoms was assessed by interview and patient diary.

Over the study period, 157 adults with laboratory-confirmed influenza and influenza subtype were enrolled—44% were positive for influenza A(H1N1), 44% for influenza A(H3N2) and 12% for influenza B. Most, about 61%, participants were male, 64% were active duty military personnel, and 72% had received influenza vaccine in the past eight months. Hospitalization occurred in almost 10% of the group.

“Seasonal influenza virus distribution among enrollees mirrored that of nationwide trends each year of study,” according to the authors, who explained that patients with A/H1N1 had upper respiratory composite scores that were lower than those with A/H3N2.

Overall, multivariate models indicated that individuals with A(H1N1) and B had increased lower respiratory symptom scores when compared to influenza A(H3N2) (A[H1N1]: 1.51 [95% CI 0.47, 2.55]; B: 1.46 [95% CI 0.09, 2.83]). Other than that, the researchers pointed out, no other differences in symptom severity scores among influenza A(H1N1), influenza A(H3N2), and influenza B infection were observed.

The study team added that influenza season—peaking in the 2012-2013 season—and female sex were found to be associated with increased influenza symptom severity.

“Our study of influenza in a cohort of otherwise healthy, outpatient adult Department of Defense beneficiaries over [five] influenza seasons revealed few differences between influenza A(H1N1), influenza A(H3N2), and influenza B infection with respect to self-reported disease severity or clinical outcomes,” researchers concluded. “This study highlights the importance of routine, active, and laboratory-based surveillance to monitor ongoing trends and severity of influenza in various populations to inform prevention measures.”


  1. Schofield C, Colombo RE, Richard SA, Chen WJ, Comparable Disease Severity by Influenza Virus Subtype in the Acute Respiratory Infection Consortium Natural History Study. Mil Med. 2020 Aug 14;185(7-8):e1008-e1015. doi: 10.1093/milmed/usaa120. PMID: 32588899; PMCID: PMC7427663.