It is also likely that the overall incidence rates in our current study have been inflated by the increased numbers of influenza admissions during the A(H1N1)pdm09 pandemic period during 2009/10. Our current rates are also higher than those of another recent Hong Kong study [7], but lower than those of an earlier report by the same group (Table 4) [3]. However, the burden of disease alters in relation to both the vaccine coverage in these children and the protection I-BET151 clinical trial elicited by

the vaccines that covered the circulating virus strain types of the respective seasons. We were unable to differentiate between cases infected with vaccine-covered or non-vaccine-covered strains as not all patients had their virus

isolates characterised. However, based on the data provided by the National Influenza Reference Laboratory (personal communications), the trivalent vaccine strains matched with our circulating strains in 2005 and 2010; and incomplete match occurred with HCS assay influenza A H3 strains for 2006 and 2011. For 2007 and 2009, the influenza A H1N1 strains were not matched while influenza B strains were not matched in 2008. However, data suggested the uptake rate among infants 6–23 months was low at 8.5% during the 2005/6 flu season [8], but the introduction of governmental subsidies to influenza vaccination for aged 6–59 months since 2008 may have improved vaccine uptake. Pregnant women are a high risk group that can benefit from seasonal influenza vaccination and recent studies have suggested that their infants will

also enjoy some degree of protection [9], [10], [11], [12], [13], [14], [15] and [16]. The vaccination uptake rate among pregnant women in Hong Kong is low in general, and ranged between 1.7 and 4.9% from various studies reported during this period [17], [18] and [19]. Should a vaccination programme targeting pregnant women also reduce the high influenza incidence of hospitalisation the in infants aged 2 months to below 6 months, it is likely that vaccine uptake would increase and cost-effectiveness of the programme would be enhanced. In contrast to high influenza hospitalisation rate in infants aged 2 months to below 6 months was the low rate in infants below two months of age (627 per 100,000). This low rate was despite the high absolute numbers of infants admitted during the first two months of life (Table 1). A US study has shown that infants below 3 months of age are more likely to present with fever alone than children aged 3 months to below 24 months of age, and although they generally do well and have a shorter duration of hospital stay, they are more likely to be admitted [20]. This analysis shows the potential of combining laboratory surveillance and passive discharge diagnosis surveillance to monitor disease burden of vaccine-preventable pathogens [1] and [2].