Hospital Acquired, Laboratory Confirmed Influenza in Ontario, Canada, 2005-09: Clinical Features, Treatment, and Outcome

Background: Many patients are admitted to acute care hospitals with influenza, but few influenza outbreaks in acute care hospitals have been described. Little is therefore known about the epidemiology of hospital-acquired influenza.

Objective: To describe the epidemiology of influenza acquired in acute care hospitals in the Toronto/Peel region (population 4 million).

Methods: Population-based surveillance for hospital-acquired influenza cases in the Toronto/Peel region has occurred since January 1, 2005. Consenting, eligible patients were enrolled using a surveillance form for data acquisition. Hospital-acquired influenza is defined as disease with onset >72 hours after hospital admission. Laboratory confirmation is defined as a positive direct test, culture or RT-PCR for influenza.

Results: From January 1, 2005 to June 12, 2009, 158 hospital acquired cases of laboratory confirmed influenza were identified. The rates in each hospital per year ranged from 0 to 0.82 per 1000 admissions, with a median of 0.21 in 2005 and 0.06 in each year of 2006 to 2008. The median age was 76.5 (range 1 month to 100 years), 79 (50.3%) were female, 14 (8.9%) required intensive care unit (ICU) admission and 22 (13.9%) died. One hundred twenty eight (81%) were influenza A, 29 (18.4%) influenza B and 1 (0.6%) influenza A/B. Of the influenza A that were typed (n=38), 12 (31.6%) were H1 and 26 (68.4%) were H3. The median number of days in the hospital prior to influenza symptoms developing were 13 (range 3 to 1117). The diagnostic test for influenza was taken a median of 1 day from symptom onset (range 0-7 days). The median number of days from symptom onset to hospital discharge was 16 days (range 0 to 753 days). Of those who had an underlying illness recorded (n=138), 126 (91.3%) had an underlying illness qualifying them for an influenza vaccine. Seventy (55.6%) had been vaccinated. Ninety four patients were treated with antivirals (93 oseltamivir; 1 amantadine). Ten (10.8%) deaths occurred in the 93 patients treated with oseltamivir and 12 (19.0%) deaths occurred in the 63 patients receiving no treatment.

Conclusions: Hospital acquired influenza results in a significant number of ICU admissions and deaths despite prompt recognition of symptoms and treatment in the in-patient setting. Adequate infection control and prevention measures are therefore crucial to avoidance of influenza acquisition in the hospital environment.