Disease burden and epidemiology of influenza among vaccine target groups

Abstract

The impact of the 2009 influenza pandemic and other recent epidemics are still being assessed. Appropriate allocation of protection and control strategies depend on accurate estimation of disease burden, with high risk groups generally being a key focus especially for distribution of influenza vaccine to maximize the disease burden prevented per vaccine when resource is limited. Therefore, better understanding the impact of influenza and control of influenza among the high risks groups with greater burden is particularly important. In this thesis, I estimated the years of life lost (YLL) associated with influenza correcting for underlying risk factors in addition to age and provided a new methodology for disease burden estimation of influenza. I focused on three vaccine target groups - healthcare workers (HCWs), cancer patients and obese people - to estimate the influenza impact and control among them using multiple approaches.

I applied a new comprehensive method to take into account the shorten life expectancy for influenza deaths with underlying risk factors compared to general population by adding excess hazards of these risk factors in the baseline life tables, finally correcting for 25% overestimation of YLL associated with 2009 pandemic.

For vaccine target groups, I analyzed the serum data from a cross-sectional study and found there was no occupation-related excess infection risk for unvaccinated HCWs following the first wave of the 2009 pandemic, supporting the effectiveness of the intensive protection and control strategies in Hong Kong. However, the reasons for the unexpectedly poor immune response observed in this study for HCWs with receipt of the 2009 pandemic influenza vaccine requires further exploration. In a large elderly cohort study with follow-up period of 1998-2012, I identified substantial impact of influenza on cancer mortality and several risk factors particularly aggravating effect of influenza on cancer mortality. With the data from this elderly cohort study, I also found that obesity was an independent risk factor for increased respiratory mortality associated with seasonal influenza. The findings from this research have provided new evidence on high risk groups who were more vulnerable to severe outcomes after influenza infection. The insights gained suggest that effective protection policy, including an influenza vaccine program, should be prudently applied for HCWs, cancer patients (especially those with certain risk factors), and obese people during epidemics and pandemics.