Thanks to Greg Folkers for sending the link to this report in JAMA: Critically Ill Patients With Influenza A(H1N1)pdm09 Virus Infection in 2014. The introduction:
The 2009 pandemic caused by influenza A(H1N1)pdm09 virus resulted in more than 18 500 reported US deaths and global estimates that were 15-fold higher.1 In contrast to seasonal influenza epidemics, during which elderly persons have the highest risk for hospitalization and death, many critically ill patients during the 2009 H1N1 pandemic were young or middle-aged adults.2 Other notable risk factors were morbid obesity and pregnancy, including the early postpartum period.2
The dominant feature of critical illness during the 2009 H1N1 pandemic was severe acute respiratory distress syndrome (ARDS) that often developed very quickly following presentation to the hospital and frequently was associated with a long and protracted course and high mortality.2
Since 2009, H1N1pdm09 virus has continued to circulate and cause critical illness worldwide, but it has not predominated in the United States until this season, with a corresponding resurgence of influenza-related hospitalizations, critical illness, severe ARDS, and deaths. This year more than 60% of laboratory-confirmed influenza-associated hospitalizations and deaths reported in adults younger than 65 years to date have been attributed to H1N1.3,4 No significant antigenic changes in circulating H1N1pdm09 virus strains compared with vaccine strains have been detected since 2009.
The relative effect on young and middle-aged adults might be partially due to their low influenza vaccine coverage and cross-reactive immunity to H1N1pdm09 virus that elderly individuals have acquired from past exposure to antigenically related viruses.
Based on lessons learned from the 2009 H1N1 pandemic, we offer suggestions for the care and management of H1N1pdm09 patients at risk for critical illness during this influenza season.