ARCHIVED - Laboratory confirmed influenza associated hospitalizations among children in the metropolitan Toronto and Peel region by active surveillance, 2004-2005


Canada Communicable Disease Report

15 September 2006

Volume 32

Number 18

A Roberts, MD (1), A Bitnun, MD (1), A McGeer, MD (2,4), D Tran, MD (1), Y Yau, MD (3), K Simpson, RN (1), K Green, MSc (2,4), DE Lowe, MD (2,4), EL Ford-Jones,MD (1,4)

  1. Division of Infectious Diseases, Department of Paediatrics, The Hospital for Sick Children,
    University of Toronto

  2. Department of Microbiology, University Health Network, University of Toronto

  3. Division of Microbiology Department of Paediatrics, The Hospital for Sick Children

  4. Toronto Invasive Bacterial Diseases Network


Influenza infection causes considerable morbidity in the pediatric population1. The high number of hospitalizations due to influenza infection has a significant impact on the health care system in Canada2. Despite these observations, there is little information about influenza-associated hospitalization rates and outcomes in Canadian children to inform vaccine policy and management recommendations. Advances in rapid laboratory-based diagnostic testing provide the opportunity to accurately assess the burden of influenza infection1. We report the hospitalization rate based on active surveillance data and describe the epidemiologic and clinical features of influenza infection in the pediatric population within the metropolitan Toronto and Peel region for the 2004-2005 season.


Data on laboratory-confirmed influenza-associated hospitalizations from 1 January, 2004, to 30 April, 2005, were extracted from two prospective active surveillance programs: the Toronto Invasive Bacterial Disease Network (TIBDN) and the Immunization Monitoring Program, ACTive (IMPACT). Within the TIBDN, all laboratories in the 13 metropolitan Toronto and Peel regional hospitals report any positive rapid antigen test or culture result for influenza, as previously published3. The results are screened for those patients who were admitted to hospital. For consenting patients, demographic and clinical data are collected by patient and physician interview as well as by chart review. Under IMPACT, 12 participating referral hospitals across Canada undertake surveillance for influenza-associated admissions. Information regarding admissions is obtained by chart review of all admissions attributable to influenza. Patients with either positive influenza culture or positive antigen test are included.

Pediatric data for the 13 regional hospitals were extracted from the TIBDN database and combined with IMPACT data on admissions to the Hospital for Sick Children (Sick Kids), Toronto, which serves as the pediatric referral centre for the region. Variables common to both datasets were retained for analysis. Patients included in the analysis were those ≤ 16 years of age who had laboratory-confirmed influenza identified by either positive immunoassay or viral culture. Excluded from the analysis were those children hospitalized for reasons unrelated to influenza and those with hospital-acquired influenza infection. The population count for postal codes within the metropolitan Toronto and Peel region, derived from the 2001 Census Report by Statistics Canada, served as the denominator in the calculation of hospitalization rates.


The total number of children ≤ 16 years old who were admitted to a hospital in the metropolitan Toronto/Peel region with influenza for the 2004-2005 season was 184. Ninety-nine (54%) were infected with influenza A and 85 (46%) with influenza B. One hundred and three (55%) were admitted to one of 13 TIBDN hospitals, and 81 (44%) were admitted to the Hospital for Sick Children (Table 1). Males accounted for 59% (n = 109) of the hospitalizations. Ninety-seven (53%) of the patients were < 24 months of age. Of these, 42 (43%) were aged 0 to 6 months and 55 (57%) were aged 6 to 24 months. The overall populationbased hospitalization rate was 0.25 per 1,000; the rate varied according to age and was highest among those < 2 years of age (0.81 per 1,000). The number of cases admitted in 2004-2005 peaked in February, the numbers being 33, 74, 64, and 13 in January, February, March and April respectively.

An underlying chronic illness was identified in 89 (48%) of the 184 children. Twenty-five percent (n = 24) of the 97 children aged 0 to 24 months and 75% (n = 65) of the 87 children > 2 years had an underlying chronic illness. Seventy-four percent (n = 60) of the 81 patients admitted to Sick Kids had an underlying chronic illness. The most common underlying conditions were reactive airway disease (24%), sickle cell and other hematologic disease (19%), malignancy (13%), neurological conditions (9%), and developmental delay (9%).

Table 1. Pediatric influenza-associated hospitalizations, age 0 to 16 years, metropolitan Toronto and Peel Region, 2004-2005 influenza season

Patient Age

No. admissions to TIBDN
hospitals (n = 13)

No. admissions to
IMPACT hospital (n = 1)

Total number
of admissions

0 to 5 mo




6 to 23 mo




2 to 4 yrs




5 to 9 yrs




10 to 16 yrs








Seventeen percent of those eligible (those ≥ 6 months of age) had received influenza vaccination prior to hospitalization. Seven (3%) were treated with antiviral medications during admission, all of whom were admitted to Sick Kids and had an underlying chronic illness. A total of 14 patients (8%) were admitted to a pediatric intensive care unit during their admission. Unfortunately, data regarding the proportion of these children with an underlying chronic illness are not available. One death was reported in a child with underlying chronic illness, although it was not felt to be causally related to influenza.


The findings of this study are consistent with previous reports indicating that younger children, particularly those < 2 years of age, have significantly higher hospitalization rates attributable to influenza than older children and adolescents4,5. The hospitalization rates of 0.25 per 1,000 for all children and 0.81 per 1,000 for children 0 to 24 months of age in our region are similar to, although somewhat lower than, those reported in the literature4. In a large cohort of children in the Tennessee Medicaid program, admission rates attributable to influenza over a 19-year period were estimated to be 10.4 per 1,000 for children < 6 months of age, 5 per 1,000 for children between 6 and 12 months of age, and 1.9 per 1,000 for children between 1 and 3 years of age4. Although the age categories are different to those in the present study, the hospitalization rates are clearly higher in the Tennessee cohort.

The observed difference in hospitalization rates may be attributed to a number of confounding factors. First, it may be related to the nature of the surveillance: a recent study confirmed that the influenza hospitalization rate as captured by a surveillance program varied with the mode and intensity of surveillance6.The rates in the Tennessee study were not derived from active surveillance but, rather, were estimated by comparing hospitalization rates during the influenza season (November through April) and non-influenza season months (May through October). The lower hospitalization rate in our study compared with that reported in Tennessee may also reflect a lower influenza infection rate within the region, a lower rate of other risk factors (e.g. smoking, household crowding), a healthier socio-economic-cultural population with greater access to earlier health care, or a higher threshold for hospitalization. Finally, the difference in our hospitalization rates may also, in part, reflect the relatively mild influenza season in 2004-2005. Using surveillance techniques similar to those in our study and hospitalization data for the same influenza season (2004-2005) in Colorado, the Centers for Disease Control and Prevention generated hospitalization rates that are comparable to ours: 1.83 per 1,000 for those 0 to 5 months, 0.6 per 1,000 for those 6 to 23 months, 0.29 per 1,000 for those 2 to 4 years, and 0.06 per 1,000 for those 5 to 17 years7.

Consistent with other reports in the literature8, the most common underlying conditions in this group of children were reactive airway disease, malignancy, sickle cell disease, and neurological conditions. Another notable observation was the infrequency of the use of antiviral medical therapy; only 3% of those hospitalized received an antiviral agent. This finding is consistent with other pediatric reports and likely reflects the uncertainty regarding the role of antiviral therapy in the treatment of hospitalized children with documented influenza infections in whom symptoms have persisted for > 48 hours9. Although there are some studies of antiviral medication given to healthy children with influenza, there are none involving high-risk10.

In contrast to the report on influenza-associated deaths among children in the United States for the 2003-2004 influenza season11, there were no deaths felt to be causally related to influenza infection in our study. This, again, may be attributed to the relatively mild influenza season captured by our study. It may also reflect variation in the socio-economic-cultural wealth of the two populations and/or differences in health care delivery between the two countries. Hence, it is interesting to note the great regional variation in rates of death as reported by Bhat et al.11: 0.001 per 1,000 in the northeastern United States and 0.0025 per 1,000 in the southern United States.

Despite the availability of a universal influenza immunization program and the relatively mild influenza season of 2004-2005, our hospitalization rates are similar to those of previous pediatric reports8,12,13. This is not unexpected given the poor overall vaccine uptake rate in the general population of < 20% according to CDC data14. From the Ontario Ministry data, the overall coverage rate (for all Ontarians) for the 2004-2005 season would be closer to 41%. This would include both the high risk and the general population. There are a number of factors specific to an influenza vaccination program that could also have a negative impact on its effectiveness. These include the reported low efficacy of currently available trivalent inactivated vaccine15 and the logistics of administering two doses of injectable vaccine to children < 9 years of age who have not been previously vaccinated. Thus, a high vaccine uptake rate may be necessary to compensate for such factors. Within this context, an understanding of modifiable barriers to influenza vaccine uptake in children, along with the implementation of strategies to minimize such barriers, may be critical to the success of a pediatric universal influenza immunization program.

In conclusion, our surveillance data indicate, as in other pediatric studies, that children < 24 months of age, those with underlying respiratory or other chronic illnesses, and those with malignancies are more likely to be hospitalized. These results are consistent with the rationale behind the current Canadian influenza vaccination recommendations for high-risk populations.


The authors acknowledge the support of IMPACT, Drs. Scheifele, Halperin, and Tam through the Public Health Agency of Canada, and the Canadian Paediatric Society in collection of data at The Hospital for Sick Children. The TIBDN thanks the many investigators, microbiology laboratory and health records staff, infection control practitioners, physicians, public health laboratory and department staff in metropolitan Toronto, and the regional municipality of Peel and committed TIBDN staff, who make surveillance at the non-IMPACT hospitals possible.


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  9. Nicholson KG, McNally T, Silverman M et al. Rates of hospitalization for influenza, respiratory synctial virus and human metapneumovirus among infants and young children. Vaccine 2006;24:102-8.

  10. Whitley RJ, Hayden FG, Reisinger KS et al. Oral oseltamivir treatment of influenza in children. Pediatr Infect Dis J 2001;20:127.

  11. Bhat N, Wright JG, Broder KR et al. Influenza-associated deaths among children in the United States 2003-2004. N Engl J Med 2005;353:2559-67.

  12. VaudryW, Roth A, Lee B et al. Active surveillance for influenza infection in children: Stollery Children's Hospital, 2003-2004 season. CCDR 2004;30:157-64.

  13. American Academy of Pediatrics Committee on Infectious Diseases. Recommendations for influenza immunization of children. Pediatrics 2004;113(5):1441-7.

  14. CDC. Childhood influenza vaccination coverage - United States, 2003-2004 influenza season. MMWR 2006;55:100-3.

  15. Yogev, R. Influenza vaccine confusion: A call for an alternative evidence-based approach. Pediatrics 2005;116:1214-5.

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