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Abstract Background Understanding the pathogenesis of influenza infection is a key factor leading to the prevention and control of future outbreaks. Pandemic 2009 Influenza H1N1 infection, although frequently mild, led to a severe and fatal form of disease in certain cases that make its virulence nature debatable. Much effort has been made toward explaining the determinants of disease severity; however, no absolute reason has been established. Results This study presents the heterogeneous virulence of clinically similar strains of pandemic 2009 influenza virus in human alveolar adenocarcinoma cells and mice. The viruses were obtained from patients who were admitted in a local hospital in China with a similar course of infection and recovered. The A/Nanchang/8002/2009 and A/Nanchang/8011/2009 viruses showed efficient replication and high lethality in mice while infection with A/Nanchang/8008/2009 was not lethal with impaired viral replication, minimal pathology and modest proinflammatory activity in lungs. Sequence analysis displayed prominent differences between polymerase subunits (PB2 and PA) of viral genomes that might correlate with their different phenotypic behavior. Conclusions The study confirms that biological heterogeneity, linked with the extent of viral replication, exists among pandemic H1N1 strains that may serve as a benchmark for future investigations on influenza pathogenesis.

Abstract Background 2009 pandemic H1N1 (pH1N1) influenza posed an increased risk of severe illness among pregnant women. Data on risk factors associated with death of pregnant women and neonates with pH1N1 infections are limited outside of developed countries. Methods Retrospective observational study in 394 severe or critical pregnant women admitted to a hospital with pH1N1 influenza from Sep. 1, 2009 to Dec. 31, 2009. rRT-PCR testing was used to confirm infection. In-hospital mortality was the primary endpoint of this study. Univariable logistic analysis and multivariate logistic regression analysis were used to investigate the potential factors on admission that might be associated with the maternal and neonatal mortality. Results 394 pregnant women were included, 286 were infected with pH1N1 in the third trimester. 351 had pneumonia, and 77 died. A PaO2/FiO2 ≤ 200 (odds ratio (OR), 27.16; 95% confidence interval (CI), 2.64-279.70) and higher BMI (i.e. ≥ 30) on admission (OR, 1.26; 95% CI, 1.09 to 1.47) were independent risk factors for maternal death. Of 211 deliveries, 146 neonates survived. Premature delivery (OR, 4.17; 95% CI, 1.19-14.56) was associated neonatal mortality. Among 186 patients who received mechanical ventilation, 83 patients were treated with non-invasive ventilation (NIV) and 38 were successful with NIV. The death rate was lower among patients who initially received NIV than those who were initially intubated (24/83, 28.9% vs 43/87, 49.4%; p = 0.006). Septic shock was an independent risk factor for failure of NIV. Conclusions Severe hypoxemia and higher BMI on admission were associated with adverse outcomes for pregnant women. Preterm delivery was a risk factor for neonatal death among pregnant women with pH1N1 influenza infection. NIV may be useful in selected pregnant women without septic shock.