Excess mortality attributable to influenza epidemics in Hong Kong

Abstract

BACKGROUND: Human influenza virus causes considerable morbidity and mortality. While deaths associated with laboratory-confirmed influenza are rare, statistical estimates of the excess deaths associated with influenza A and B virus infections tend to identify a much more substantial burden of disease. We explored the excess mortality associated with influenza types and subtypes from different underlying causes in Hong Kong, a subtropical city with influenza activity through much of the year. MATERIALS AND METHODS: We applied multiple linear regression models to fit age-specific all-cause and cause-specific mortality rates in Hong Kong from 1998 through 2009, adjusting for influenza virus activity as the product of influenza-like illness rates from sentinel surveillance and laboratory detection rates of influenza types and subtypes. Models were also adjusted for the activity of cocirculating respiratory syncytial virus, environmental temperature and absolute humidity, and periodic temporal trends in mortality rates. The differences between estimated mortality rates in the presence or absence of recorded influenza activity were used to estimate influenza-associated mortality. RESULTS: The annual influenza-associated all-cause excess mortality rate was 10.9 (95% CI, 8.1-13.8) per 100,000 person-years, which was an average of 742 (95% CI, 552-933) excess deaths each year from 1998 through 2009. Most (95%) of the excess deaths associated with influenza occurred in the elderly. More influenza-associated excess mortality was estimated to occur in deaths from respiratory (53%) than cardiovascular (18%) causes. Influenza A (H3N2) epidemics were associated with more deaths than epidemics of other types or subtypes during the study period. CONCLUSIONS: Influenza was associated with a substantial number of deaths each year particularly among the elderly in Hong Kong in the past decade. Respiratory diseases accounted for more influenza-attributable deaths than cardiovascular diseases. The proportion of influenza-attributable excess deaths caused by cardiovascular diseases was lower in Hong Kong than in the United States. Influenza A (H3N2) was associated with higher excess mortality than other seasonal influenza subtypes.