Elderly hit hardest by seasonal influenza

**CDC. MMWR. 2010;59:1057-1062.**

People aged 65 years and older remain at the highest risk of morbidity and mortality from seasonal influenza, according to the CDC.

In a report published online today, CDC researchers updated previous mortality and morbidity data on influenza seasons between 1976 to 2007.

“Many deaths associated with influenza infections occur from secondary infections such as bacterial pneumonia or complications of chronic conditions such as congestive heart failure and chronic obstructive pulmonary disease,” the CDC researchers wrote. “Therefore, estimates using underlying respiratory and circulatory mortality data (which include pneumonia and influenza causes) can provide an upper bound for influenza-associated deaths.”

The estimated overall annual average of influenza-associated deaths with underlying pneumonia and influenza causes was 6,309. The average annual influenza-associated death rate was 2.4 deaths per 100,000, health officials noted. Adults aged 65 years and older accounted for 87.9% of the overall estimated annual average of influenza-associated deaths with these underlying causes. Among this age group, an estimated average of 5,546 deaths occurred annually during the study period, and the researchers found an estimated annual rate of 17 per 100,000 for influenza-associated deaths with underlying pneumonia or influenza causes.

For adults aged 19 to 64 years, the estimated annual average was 666 for influenza-associated deaths associated with pneumonia. The average annual rate of influenza-associated deaths was 0.4 per 100,000, the researchers said.

Data also indicated that, during 1976 to 2007, the estimated overall annual average of influenza-associated deaths with underlying respiratory and circulatory causes — including pneumonia and influenza causes — was 23,607, with an estimated average annual rate of nine deaths per 100,000.

Again, influenza-associated deaths with these underlying causes took the biggest toll on people aged 65 years and older, the researchers said, with deaths occurring at an estimated rate of 66.1 per 100,000 per year. Results also indicated that the estimated annual average of deaths in this age group was 21,098. At 89.4%, deaths in this age group were also responsible for the majority of the overall annual average.

An estimated average of 2,385 influenza-associated deaths with these underlying causes occurred among adults aged 19 to 64 years, and the average annual rate was 1.5 deaths per 100,000.

Pediatric data

Children aged younger than 19 years, however, experienced the lowest average for influenza-associated deaths associated with underlying pneumonia. The estimated annual average was 97, and the average annual rate was 0.1 deaths per 100,000 for this age group. Also in the pediatric group, there was an estimated annual average of 124 influenza-associated deaths occurring with underlying respiratory and circulatory causes. The average annual rate, 0.2 deaths per 100,000, also remained low.

During the 22 seasons when influenza A (H3N2) dominated, the average mortality rates were 2.7 times higher, the researchers said, compared with other seasons.

Analysis also showed that 8.5% of all pneumonia and influenza deaths and 2.1% of all respiratory and circulatory deaths were related to influenza.

The researchers noted that this study’s results were comparable to others purporting estimates on influenza-associated deaths, especially regarding age distribution. They pointed out, however, that certain limitations precluded them from properly ascertaining the effect on young people.

“Future research that considers years-of-life-lost is needed to better communicate the mortality burden of influenza in these younger populations,” the researchers wrote. They also acknowledged that breakdown of data by demographic and health risk groups will also be important to understanding the disease.

Prevention is key, the researchers said. “Annual influenza vaccination is the best way to reduce the risk for complications from influenza infections and is now recommended for all persons aged older than 6 months in the United States.”

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