For severe, critical and fatal cases of 2009 H1N1 influenza in the winter in the Northern Hemisphere, the detailed features are not fully known. The aim of this study was to examine these features through describing these cases in Beijing, China in 2009.

Data on severe, critical and fatal cases were collected via the Notifiable Disease Surveillance System and a designated surveillance system for managing 2009 H1N1 influenza cases in Beijing. The characteristics and risk factors of these cases were elucidated.

A total of 475 severe cases, 73 critical ones and 69 deaths were identified in 2009. The proportion of obesity was low, as well as pregnancy. About half of them had no underlying disease. Most of deaths had multi-organ failure, with a median interval from illness onset to death of ten days. Delay in visiting hospital, cardiovascular disease and allergy predicted a higher risk of severe disease, and cases aged 6–17 years were at lower risk. Cases not promptly receiving neuraminidase inhibitors were at increased risk of death.

Age and underlying disease are significantly associated with severity of outcomes of 2009 H1N1 influenza; prompt presentation to hospital and use of neuraminidase inhibitor were protective.