Excess mortality associated with influenza A and B virus in Hong Kong, 1998-2009

Abstract

Background: Although deaths associated with laboratory-confirmed influenza virus infections are rare, the excess mortality burden of influenza estimated from statistical models may more reliably quantify the impact of influenza in a population.

Methods: We applied age-specific multiple linear regression models to all-cause and cause-specific mortality rates in Hong Kong from 1998 through 2009. The differences between estimated mortality rates in the presence or absence of recorded influenza activity were used to estimate influenza-associated excess mortality.

Results: The annual influenza-associated all-cause excess mortality rate was 11.1 (95% confidence interval [CI], 7.2-14.6) per 100,000 person-years. We estimated an average of 751 (95% CI, 488-990) excess deaths associated with influenza annually from 1998 through 2009, with 95% of the excess deaths occurring in persons aged ≥65 years. Most of the influenza-associated excess deaths were from respiratory (53%) and cardiovascular (18%) causes. Influenza A(H3N2) epidemics were associated with more excess deaths than influenza A(H1N1) or B during the study period.

Conclusions: Influenza was associated with a substantial number of excess deaths each year, mainly among the elderly, in Hong Kong in the past decade. The influenza-associated excess mortality rates were generally similar in Hong Kong and the United States.

Figure 1.

(A) Weekly influenza activity (black line), weekly all-cause mortality (gray line), and 19 influenza seasons (gray areas); (B) weekly respiratory syncytial virus (RSV) activity; (C) averaged weekly mean temperature; and (D) averaged weekly mean absolute humidity, in Hong Kong, 1998–2009. Influenza activity was estimated by the weekly proportion of consultations for influenza-like illness at sentinel clinics multiplied by the weekly influenza detection rate in the local public health laboratory. Influenza seasons were defined as periods of at least 2 consecutive weeks with influenza activity exceeding 0.005 (gray dashed line). RSV activity was estimated by the weekly proportion of consultations for influenza-like illness at sentinel clinics multiplied by the weekly RSV detection rate in the local public health laboratory.

Figure 2.

Breakdown of the estimated annual influenza-associated excess mortality rates (95% confidence intervals) from 7 major causes and all causes in Hong Kong from 1998 through 2009. Separate regression models were fitted for 4 subcategories of major causes, 7 major causes, the sum of the 7 major causes, other causes, and for all-cause mortality, with good consistency between the estimates. “Other causes” refers to the causes of death other than the 7 major causes shown in the Figure. The time series of deaths from other causes was derived by subtracting deaths from the 7 major causes from the all-cause deaths.