Rates of Hospital-Acquired Influenza Due to the Pandemic H1N1 Virus in 2009, Compared with Seasonal Influenza

Abstract

During a 9-week period (18 May to 19 July 2009), data on 54 cases of pandemic H1N1 influenza that were confirmed by polymerase chain reaction were prospectively collected, and were compared with 95 proven cases of seasonal influenza A in inpatients that had been registered prospectively during the influenza seasons of 1999 (55 cases) and 2004 (40 cases). During the 2009 pandemic H1N1 influenza season, all patients were managed in agreement with Chilean guidelines, using droplet and contact precautions and antiviral therapy with the neuraminidase inhibitors oseltamivir or zanamivir. Pandemic influenza affected fewer older patients. In fact, 31.5% of patients with pandemic H1N1 influenza were over 65 years old, as compared with 68% of patients with seasonal influenza (P<0.001). A large proportion of patients with chronic diseases and a relatively low number of immunocompromised subjects were seen in both groups. Pregnant women hospitalized with influenza were seen only during the 2009 H1N1 influenza outbreak. There were twice as many cases of hospital-acquired H1N1 influenza as there were of seasonal influenza. With respect to clinical symptoms, fewer patients with pandemic H1N1 influenza had fever and bronchial obstruction, compared with patients with seasonal influenza; on the other hand, more cases of pneumonia, more admissions to an intensive or intermediate care unit, and more deaths were observed among the patients with H1N1 influenza. It is concluded that H1N1 infection affected predominantly the patients who were in a younger age group or pregnant; that the group with H1N1 influenza had a similar rate of comorbidities, a lower rate of severe influenza symptoms and significant laboratory findings, and a higher frequency of intensive care admission, pneumonia, and death; and that there was an elevated rate of nosocomial acquisition despite the implementation of infection control measures.