Influenza Season Hospitalization Trends in Israel: A Multi-Year Comparative Analysis 2005/2006 Through 2012/2013

Hospitalization Days

The mean monthly rate of hospitalization days per 100,000 residents showed a similar trend to that of the hospitalization rates (panels A, B, and C in Figure; Supplementary Table 2), with the most prominent increases observed among infants and children <5 years and adults ≥65 years.

The mean monthly rate of hospitalization days per 100,000 during influenza seasons for all ages due to all diagnoses was 8% higher (P < 0.001) as compared with the summer seasons (panel A in Figure; Supplementary Table 2). Statistically significant increases were also found among patients diagnosed with respiratory/cardiovascular diseases and for influenza/pneumonia (panels B and C in Figure; Supplementary Table 2).

Children <5 years of age showed the largest increases during the influenza season as compared with the summer, with an up to 155.9% increase in the mean monthly rate of hospitalization days due to influenza/pneumonia (panel C in Figure; Supplementary Table 2), and an up to 206.6% increase for respiratory/cardiovascular diseases in infants <1 year of age (panel B in Figure; Supplementary Table 2). In adults, the largest increases were observed among those ≥75 years; the rates for influenza/pneumonia increased by about 40% (panel C in Figure; Supplementary Table 2), and the rates for respiratory/cardiovascular diseases increased by 14.8%-20.7% as compared with the summer months (panel B in Figure; Supplementary Table 2).

Statistically significant decreases in monthly mean rate of hospitalization days during influenza seasons were observed for all diagnoses in the 5-54 age groups (panel A in Figure; Supplementary Table 2). Decreases were not seen for the diagnoses of respiratory/cardiovascular diseases or influenza/pneumonia (panels B and C in Figure; Supplementary Table 2).

Hospital Length of Stay

The longest mean monthly LOS due to all diagnoses (for both influenza and summer seasons) was observed in adults ≥65 years of age (Table 2). The longest mean monthly LOS due to influenza/pneumonia (for both influenza and summer seasons) was observed in adults ≥55 years or older, and for the diagnosis of respiratory/cardiovascular diseases, infants <1 year and adults ≥55 years had the longest LOS.

The differences between influenza and summer seasons in mean monthly LOS were mostly small or not observed in any of the diagnostic categories examined. The mean monthly LOS due to a diagnosis of influenza/pneumonia was shorter during the influenza seasons than summer seasons in most age groups. These differences were statistically significant in children <5 years and adults ≥45 years (Table 2).

The mean LOS due to respiratory/cardiovascular diseases was significantly shorter during influenza seasons than summer seasons in children under 5.

Bed Occupancy

Mean bed occupancy was significantly higher during influenza seasons compared with the preceding summer seasons, both in internal medicine and pediatric departments (Table 3). The differences were higher in pediatric departments as compared with internal medicine departments for most years evaluated.

Our study demonstrates trends of excess hospitalizations during influenza as compared with summer seasons and identifies patient groups that contribute mostly to changes in hospital burden between these seasons.

Overall, the present study demonstrates differences between influenza and summer seasons for all measures tested: hospitalizations, hospitalization days, LOS, and bed occupancy. These differences were due primarily to excess number of hospitalizations and hospitalization days, rather than to longer LOS.

Our results concerning hospitalizations for all diagnoses are consistent with a United States report showing about 5% more hospitalizations following emergency department visits during winter compared with summer.¹²

The increase in hospitalizations and total hospitalization days in older age groups reflects the probability of severe diseases in a population with multiple comorbidities, and is consistent with a 90% influenza-related mortality due to respiratory and cardiovascular diseases reported in patients 65 and older.¹³ The increase in hospitalization and total hospitalization days in the age groups <5 years during influenza seasons are consistent with studies showing that the risk of children to contract influenza is higher than that of adults surrounding them. In this regard, outbreak investigations during the 2009 influenza pandemic showed that influenza attack rates in children were higher than those of adults.¹⁴

Nationwide studies from Singapore and Taiwan also showed more hospitalizations related to influenza in young children and older adults.^15,16

The increase in hospitalization days for all patients should be interpreted while taking into account the mean monthly LOS per patient (Table 2). In most age groups, a small decrease in the mean LOS for individual patients with the diagnosis of influenza/pneumonia was observed (Table 2). This decrease may suggest a need to shorten hospitalization slightly in order to accommodate new patients. Similarly, the decrease in hospitalization rates from all diagnoses during influenza seasons in the 5-54 years age groups (Figure) may stem, at least in part, from the shortage of available hospital beds due to patient overload. Additional study is required to further explore these decreases and their possible effects on morbidity and mortality.

Influenza vaccine guidelines in Israel following the 2009 influenza pandemic recommend influenza vaccination for all individuals age 6 months and older. However, influenza vaccination in Israel has remained low. Specifically, vaccination rates among children below the age of 5 years have been approximately 21%, as compared with 60%-65% in adults 65 years and over.¹⁷ Given the low rate of vaccination in children, we believe that there would be minimal or no difference in hospitalization of children under the age of 5 years, between the pre- and postpandemic years. Israel has started a school-based influenza vaccination program for the 2016-2017 influenza season in an effort to increase childhood influenza vaccination. It would be important to see if the expansion and continuation of the program would have an effect on influenza season hospitalizations.

Our study has several advantages. To the best of our knowledge, it is the first study examining differences in hospital burden between influenza and summer seasons on a national level. As such, it constitutes one of the largest studies on the subject. In addition, our study relies on original data, rather than estimates. Analysis of specific months of each year in which influenza virus circulates provides a targeted analysis of influenza seasons, rather than the entire winter season. The comparison with summer months is of great importance for preparatory plans by health systems, as it takes into account the degree of variation between the seasons. The analysis of 6 influenza seasons in our study intended to take into account season-to-season disease variability. Such variability among influenza seasons has been described previously due to changes in the virus itself, the population immune status, and the weather.¹⁸

We used several diagnosis categories to evaluate different aspects of hospital burden. Although the category of “all diagnoses” provided a broad assessment of hospital burden, influenza/pneumonia or pulmonary/cardiovascular disease constituted a more specific measure of influenza-associated burden.

Evaluating LOS added to the accuracy of hospital burden estimates, and our age-group analysis highlighted the specific age groups responsible for changes in hospital burden. Thus, the use of several measures to assess influenza season morbidity provides a comprehensive picture of the hospitalization dynamics between influenza and summer seasons. In this regard, the trends observed in our study for hospitalizations and total hospitalization days correspond to those observed in bed occupancy, especially for hospitalization rates due to all causes.

Our study has several limitations. We did not rely on laboratory diagnosis of influenza to determine burden. Because obtaining specimens for viral detection is usually based on individual clinical judgement, and patients hospitalized with influenza-related complications can often test negative for the virus due to time elapsed from disease onset, relying on a laboratory-based analysis may lead to underestimation of hospital burden. On the other hand, it is possible that patients with morbidity not specifically related to influenza were included in our analysis. Respiratory syncytial virus (RSV), for example, can also cause respiratory illness during the fall and winter.¹⁹ However, in Israel, RSV epidemic usually occurs before the influenza epidemic.^17,20 Thus, it is expected that only a small percentage of hospital admissions due to RSV would occur during the influenza season. Another limitation of our study relates to the small number of months in the beginning and end of influenza seasons in which influenza activity was recorded only during part of the month. Thus, hospital burden may have been underestimated during these “incomplete” months. Future studies using time series analysis methods will contribute to a more accurate estimation of such differences, as well as account for variability in influenza activity.

Our results clearly highlight the issues that challenge hospitals in Israel, and possibly other countries, during influenza seasons, such as the most affected age groups and the shortening of hospital stay. Thus, our findings are most relevant for hospital preparedness towards influenza seasons, particularly in terms of the need for additional hospital beds and personnel.