However even with a practice of routine NPA testing for respiratory related illness, not

all children will have specimens collected for laboratory confirmation. In our analysis we have made estimates of possible increased disease burden had all children had specimens taken. The laboratory surveillance at PWH suggested that up to 1.6% of infants aged above 6 days and below 6 months of age and 5.2% of children Z-VAD-FMK solubility dmso aged above 6 days to below 18 years are admitted to hospital as a result of influenza infection. We adjusted the CMS flu diagnosis estimates using factors derived from linking our laboratory surveillance results at PWH to the CMS coded diagnoses and then extrapolated these adjustments to the whole of Hong Kong. These adjusted rates were generally higher than the unadjusted rates (Fig. 2 and Fig. 3). During the A(H1N1)pdm09 pandemic in 2009/10 the proportion of children aged above 6 days to below 18 years admitted to hospital who had a diagnosis of influenza almost doubled (9.8%). Reasons for this increase incidence during 2009/2010 IOX1 could reflect a genuine increase in disease burden or alternatively

it could reflect changes in admission policy e.g. all suspected A(H1N1)pdm09 infections, including mild cases, were recommended for admission. Measures for severity of illness in the current study were length of stay, intensive care unit admission and outcome. Severity of influenza as measured by mortality mafosfamide and

length of stay did not appear to be greater in the 6M group as compared to the 18Y group. The median length of stay for the A(H1N1)pdm09 admissions was similar to the that of the non-A(H1N1)pdm09 influenza admissions (Appendix 12) but when categorised into groups, a greater proportion of children with A(H1N1)pdm09 had a length of stay less than 2 days (Table 3), possibly reflecting less severe disease or a greater proportion of admissions with mild disease. However the number of intensive care unit admissions with any CMS diagnosis of influenza was highest during 2009/10. Incidence estimates based on adjustment factor 3 (PWH laboratory confirmed influenza rate) tended to be higher than the other incidence estimates except during 2009/10 (Fig. 2), possibly reflecting a sustained high level of routine NPA testing for influenza during the whole study period at PWH, but with other HA hospitals only increasing their NPA testing for influenza from 2009/10. Limitations to our incidence estimates include a number of assumptions related to admissions to public HA hospitals and the resident Hong Kong population. The proportion of admissions to public hospitals has fallen in recent years and there has been a marked increase in the number of mothers from mainland China delivering in Hong Kong.