9) and y is the admissions to public HA hospitals as a percentage of total admissions by age (Appendix 1). These proportions were weighted by the number of admissions when incidence estimates were calculated for different age groups: ∑j(Admissionsj×Pj)∑jAdmissionsjwhere Admissionsj is the number of admissions in the jth age group, and Pj is the proportion of admissions to HA hospitals find more in the jth age group; z is the estimated resident population by age (Appendix 2). Incidence rates were calculated by monthly age

groups and then re-grouped according to different age ranges (Table 1). Since a CMS flu diagnosis may reflect both under- and over-diagnosis, we applied adjustment factors to this CMS Flu derived incidence estimate (Table 1). These factors were derived by linking the PWH laboratory surveillance data (LAB flu+ or LAB flu−) with the PWH CMS data (CMS flu+ or CMS flu−) (Appendix 3). The first factor was derived to adjust for potential under-reporting of influenza infection by the CMS system. The second factor was derived to reflect the potential under-estimation of a PWH laboratory diagnosis of influenza by accounting for the fact that not all admissions with a primary respiratory-associated diagnosis had a NPA specimen sent to the laboratory for testing. The third factor was the EGFR inhibitor proportion of all admissions to PWH by age group

that had a laboratory confirmed diagnosis of influenza. No assessment or adjustment was made for possible nosocomial infections. During the 6-year study Bay 11-7085 period 1 April 2005 to 31 March 2011, there were 624,916 children admitted to the paediatric medical wards of all HA hospitals; 2 had no gender specified and 86 had missing age data and were excluded. Of the 624,828 children with valid data, 94.5% (590,683) were below the age of 18 years and 32.9% (205,783) were below the age of

6 months, 13.9% (86,582) were aged above 6 days to below 6 months (6M group) and 75.5% (471,482) were aged above 6 days to below 18 years (18Y group). In the 6M and 18Y groups respiratory-associated disorders were respectively coded as the primary diagnosis in 13.9% and 27.2% of admissions, and as the primary or as one of any 9 secondary diagnoses in 15.7% and 31.8% (Appendix 4). The percentage of all discharges with a primary diagnosis and “any” diagnosis of influenza (CMS flu) ranged from 0.3% to 1.4% and 0.4 to 1.9% in the 6M group and from 0.9% to 4.2% and 1.3% to 6.0% in the 18Y group respectively in the 12 HA hospitals (Appendix 5). Likewise rates of admissions coded as having a respiratory illness varied considerably between these different hospitals. Influenza admissions peaked during February and September (Fig. 1). Over the full 6 year study period there was a peak of admissions during the April 2009–March 2010 (Fig. 1). A similar pattern was seen with the data from all HA hospitals and with data from PWH alone (Fig. 1).