Observational cohort study of influenza A/H1N1 pneumonia outcomes in Vietnam.

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medRxiv Published April 23, 2026 Medically reviewed April 25, 2026 Study authors: Ho, M. Q.; Duong, T. B.; Nguyen, T. L. N.; Tri, N. S.; Bui, T.; Thai, T. T.; Muscatello, D. J.; Sunj… DOI ↗ By Dr. Amelia Tan, PhD · Internal Medicine & Chronic Disease

Key Takeaway

Consider monitoring older patients and those with comorbidities or ARDS for complications in H1N1 pneumonia.

This is a retrospective cohort study from a leading infectious diseases center in Vietnam, examining outcomes in 158 adult patients with influenza A(H1N1)pdm09 pneumonia among 729 admissions from 2009-2019. The authors synthesized data on complications and predictors.

Key findings include an ICU admission rate of 48.7% (77/158; 95% CI 41.1-56.5%), a mortality rate of 8.2% (13/158; 95% CI 4.9-13.6%), moderate-to-severe ARDS in 36.7% (58/158), and invasive ventilation in 15.2% (24/158). Predictors of ICU admission included age >60 years (adjusted odds ratio [AOR] 13.864; 95% CI 2.185-87.956, P=0.005), comorbidities (AOR 6.527; 95% CI 1.710-24.915, P=0.006), and moderate-to-severe ARDS (AOR 14.027; 95% CI 4.220-46.627, P<0.001). Predictors of mortality included invasive ventilation (AOR 55.355; 95% CI 1.486-2062.375, P=0.030) and double-dose oseltamivir or combination therapy (AOR 32.625; 95% CI 1.594-667.661, P=0.024).

The authors note limitations, including the single-center setting in Vietnam, a retrospective design from medical records, and no control group for comparison. The study is observational, so associations are reported, not causation.

Practice relevance suggests monitoring older patients and those with comorbidities, liver enzyme elevation, or moderate-to-severe ARDS for timely detection of complications requiring intensive care. Certainty is limited by the study design.

Study Details

EvidenceLevel 5

PublishedApr 2026

View Original Abstract ↓

The A(H1N1)pdm09 virus remains a major global health threat. This study examined the burden of ICU admission, mortality, and associated predictors among patients with A(H1N1)pdm09 pneumonia in a leading center for infectious diseases in Vietnam. Information on demographic, clinical, and laboratory characteristics, and outcomes was retrieved from medical records of adults admitted with influenza A(H1N1)pdm09 during 2009-2019. Among 729 cases, 21.7% (158/729) developed pneumonia. Among 158 pneumonia cases, 36.7% (58/158) developed moderate-to-severe acute respiratory distress syndrome (ARDS), and 15.2% (24/158) received invasive ventilation. ICU admission and mortality rates were 48.7% (77/158, 95%CI 41.1-56.5%) and 8.2% (13/158, 95%CI 4.9-13.6%), respectively. Predictors of ICU admission included being >60 years old (adjusted OR [AOR] 13.864, 95%CI 2.185-87.956, P=0.005), comorbidities (AOR 6.527, 95%CI 1.710-24.915, P=0.006), AST (AOR 1.013, 95%CI 1.001-1.025, P=0.029), and moderate-to-severe ARDS (AOR 14.027, 95%CI 4.220-46.627, P<0.001). Predictors of mortality were invasive ventilation (AOR 55.355, 95%CI 1.486-2062.375, P=0.030) and double-dose oseltamivir or combination therapy (AOR 32.625, 95%CI 1.594-667.661, P=0.024). In conclusion, mortality is not rare in A(H1N1)pdm09 infection. Monitoring of older patients and those with comorbidities, liver enzyme elevation, or moderate-to-severe ARDS is essential for the timely detection of complications requiring intensive care.