Flu Fatalities, the Full Field: Influenza Associated Deaths Across the Age Spectrum-Los Angeles County, California, 2009-2014

BACKGROUND:  Currently, states are only required to report influenza-associated deaths (IADs) in the pediatric population. Some jurisdictions additionally monitor influenza deaths in a limited adult population. More often, the elderly population is excluded from IAD surveillance even though they are thought to account for the majority of influenza mortality. Los Angeles County Department of Public Health (LACDPH) tracks fatalities of all age groups for our population of over 10 million. These data provide means to track changes in mortality from season to season and help identify high risk groups that would benefit from future prevention methods aimed at decreasing morbidity.

METHODS:  To assess the influenza-attributed mortality in Los Angeles County (LAC), LACDPH compared IADs of all age groups and dominant strain type from the past five seasons (September 2009-September 2014). During the 2009 pandemic, heightened surveillance measures were implemented including reporting of influenza deaths of all ages. Once the emergency situation was over, LACDPH revised reporting requirements. As of October 2010 intensive care cases were no longer reportable. However, all influenza-related fatalities remained reportable within seven days of identification. A confirmed influenza death is defined by a positive lab test, compatible symptoms, and clear progression from illness to death. Laboratory confirmation tests include rapid antigen testing, polymerase chain reaction (PCR), direct-fluorescent antibody staining, or viral culture. Reports are submitted year-round from the following sources: 1. Hospitals and providers, 2. LAC Coroner Office, 3. LACDPH morbidity unit death certificate analysis. 

RESULTS:  During three of the past five flu seasons (2009-10, 2010-11, and 2013-2014), IADs among the 18-64 year old age group, which accounts for over 60 % of the population in LAC, was substantially higher than both the <18 and the ≥65 years groups and influenza A pH1N1 was the dominant strain.  During the other two seasons where H1N1 was not predominant, the majority of fatalities occurred in the ≥65 age group. 

CONCLUSIONS:  LAC data shows a clear difference in mortality burden by age group depending on the dominant circulating strain. Previous studies report that the majority of IADs occur among adults 65 years and older. However, LAC data show conflicting trends over the past five seasons with most deaths occurring among 18-64 year old age group, specifically during A pH1N1 seasons. Expanding surveillance of influenza deaths to include all age groups provides more robust data that is useful for public health prevention messages and interventions.