Influenza Season Hospitalization Trends in Israel: A Multi-Year Comparative Analysis 2005/2006 Through 2012/2013
BACKGROUND: Influenza-related morbidity impacts healthcare systems, including hospitals.
OBJECTIVE: To obtain a quantitative assessment of hospitalization burden in pediatric and internal medicine departments during influenza seasons compared with the summer months in Israel.
METHODS: Data on pediatric and internal medicine hospitalized patients in general hospitals in Israel during the influenza seasons between 2005 and 2013 were analyzed for rate of hospitalizations, rate of hospitalization days, hospital length of stay (LOS), and bed occupancy and compared with the summer months. Data were analyzed for hospitalizations for all diagnoses, diagnoses of respiratory or cardiovascular disease (ICD9 390-519), and influenza or pneumonia (ICD9 480-487), with data stratified by age. The 2009-2010 pandemic influenza season was excluded.
RESULTS: Rates of monthly hospitalizations and hospitalization days for all diagnoses were 4.8% and 8% higher, respectively, during influenza seasons as compared with the summers. The mean LOS per hospitalization for all diagnoses demonstrated a small increase during influenza seasons as compared with summer seasons. The excess hospitalizations and hospitalization days were especially noticed for the age groups under 1 year, 1-4 years, and 85 years and older. The differences were severalfold higher for patients with a diagnosis of respiratory or cardiovascular disease and influenza or pneumonia. Bed occupancy was higher during influenza seasons compared with the summer, particularly in pediatric departments.
CONCLUSIONS: Hospital burden in pediatric and internal medicine departments during influenza seasons in Israel was associated with age and diagnosis. These results are important for optimal preparedness for influenza seasons.
© 2017 Society of Hospital Medicine
Influenza-associated morbidity poses a significant hospital burden.1 A study from the United States estimated that seasonal influenza is responsible for 3.1 million hospitalization days per year.2
Assessment of hospital burden during influenza seasons presents a challenge due to several possible factors, such as inaccurate recording of diagnosis3 and incomplete age group data. Although great emphasis has historically been placed on older age groups, a study from England and Wales showed that the number of hospitalizations and deaths resulting from influenza was significantly higher in children as compared with adults.4 Moreover, excess visits to emergency departments in New York City because of fever and respiratory morbidity during influenza seasons were found mostly among school-age children, whereas in adults, the surplus was small to nonexistent.5
Studies examining influenza-related hospitalizations evaluated numbers and rates of hospitalization.6-11 However, information regarding length of hospitalizations, hospitalizations during the influenza season that were not influenza related, or comparisons between influenza seasons and summer seasons is scarce. These determinants are of great importance for hospital preparedness towards influenza seasons. The aim of the current study was to estimate excess hospitalizations and length of hospitalization during influenza seasons, as compared with the summer, in different age groups and selected diagnoses in Israel.
METHODS
Data Sources
Hospitalization data of internal medicine and pediatric departments in 28 acute care hospitals in Israel between 2005 and 2013 were obtained from the National Hospital Discharges Database managed by the Health Information Division (HID) in the Israel Ministry of Health (MOH). The information included number of discharges (including in-hospital deaths), number of hospitalization days, and the mean length of stay (LOS) per discharge for all diagnoses and for primary or secondary diagnoses of respiratory/cardiovascular disease (ICD9 390-519) and influenza/pneumonia (ICD9 480-487).
Bed occupancy rates for internal medicine and pediatric departments were based on the National Patient Flow Database managed by the HID.
The 2009-2010 pandemic influenza season was excluded from analysis due to different morbidity patterns and timing (April 2009 until August 2010) as compared with seasonal influenza.
Data Classification
Hospitalizations data were analyzed for all ages, for specific age groups (the first year of life [0], ages 1-4, 5-14, 15-24, 25-34, 35-44, 45-54, 55-64, 65-74, 75-84, and 85 years and older), for all diagnoses, and for primary or secondary discharge diagnosis of respiratory/cardiovascular disease (ICD9 390-519) and influenza/pneumonia (ICD9 480-487).
Duration of Influenza Season
The beginning and the end of the influenza season were determined by the National Influenza surveillance program, which includes on average 22 community sentinel clinics, throughout Israel, each influenza season. These clinics send nose-throat samples from a convenience sample of patients with influenza-like illness (ILI), from week 40 of each year until the end of the influenza season in the subsequent year. These samples are analyzed for the presence of influenza virus by real-time reverse transcription polymerase chain reaction (RT-PCR) at the Central Virology Laboratory of Israel. Based on influenza virus detection in nose-throat samples from patients with ILI attending the community sentinel clinics, we determined the first and last month of each influenza season. The first month in which positive influenza samples were identified in sequence was defined as the first month of the season. The month in which the sequence of positive influenza samples stopped was defined as the last month of the season.
The 2009-2010 pandemic influenza season was excluded from analysis due to different morbidity patterns and timing (April 2009 until August 2010) as compared with seasonal influenza.
Data Analysis
Rates. Rates of monthly hospitalizations and monthly hospitalization days were calculated per 100,000 residents for all ages and for the specific age groups. Estimated average population sizes in different years for all ages and for specific age groups were obtained from the Central Bureau of Statistics (https://www.cbs.gov.il/reader/shnaton/templ_shnaton.html?num_tab=st02_01&CYear=2014). Monthly LOS was not converted to rates.
