CDC: Annual Influenza Mortality Estimates Vary Substantially

Annual estimates of influenza-associated deaths from 1976-2007 varied substantially by season, influenza virus type, underlying cause of death, and age group, according to revised statistical models, the Centers for Disease Control and Prevention reported August 27 in its Morbidity and Mortality Weekly Report.

The “incredible variation” indicates that using a single, average estimate insufficiently communicates the mortality burden of influenza, Dr. David Shay, medical officer with the CDC’s National Center for Immunization and Respiratory Diseases, said in a media briefing.

With a low of 3,349 estimated deaths in 1986-87 and a high of 48,614 deaths in 2003-2004, the estimated annual rate of influenza-associated deaths in the United States from 1976-2007 ranged from 1.4 to 16.7 deaths per 100,000 persons, according to the new models, which update the CDC’s previously published estimates for 1976-2003 and include new data from 2006-2007.

Because of the wide variability across influenza seasons, “it is relatively meaningless to try to summarize [influenza burden] with one number,” Dr. Shay stressed. “There are very few average seasons, so providing a simple average fails to communicate the impact of flu in an understandable fashion.”

For this reason, the CDC advises quantifying influenza-associated deaths in the context of circulating virus strains and underlying causes of death among age groups. Toward this end, the influenza-associated mortality estimates in the CDC’s revised models are provided for three age groups (younger than 19 years, 19–64 years, and 65 years or older) and for two categories of underlying cause of death codes: pneumonia and influenza causes and respiratory and circulatory causes.

For pneumonia and influenza causes, the respective estimated annual average of influenza-associated deaths and the rate of influenza-associated deaths per 100,000 people were 6,309 and 2.4 for the U.S. population overall; 97 and 0.1 for persons younger than 19 years; 666 and 0.4 for adults age 19-64 years; and 5,546 and 17.0 for adults 65 years and older, the report states.

For deaths with underlying respiratory and circulatory causes, the respective estimated number and rate of influenza-associated deaths per 100,000 was 23,607 and 9.0 for the U.S. population overall; 124 and 0.2 for persons younger than age 19 years; 2,385 and 1.5 among adults age 19–64 years; and 21,098 and 66.1 among adults age 65 years and older (MMWR 2010;33:1057-62).

For both causes, “the average mortality rates for the 22 seasons during which influenza A(H3N2) was a prominent strain were 2.7 times higher than for the nine seasons that it was not,” the authors reported. “The average annual number of influenza-associated deaths during influenza A(H3N2) prominent seasons was 7,722 for pneumonia and influenza causes and 28,909 for respiratory and circulatory causes, compared with 2,856 deaths for pneumonia and influenza causes and 10,648 deaths for respiratory and circulatory causes in seasons in which it was not.”

The numbers “confirm that influenza has a substantial burden of mortality each year, but also that the burden can change substantially from year to year,” Dr. Shay stated.

The findings represented in the revised model are limited by a number of factors, including the failure to account for co-circulating pathogens such as respiratory syncytial virus; the possibility that changing virus surveillance data may reduce the relevance of comparing estimates over time; and the possibility that the increase in the number of adults older than age 65 years during the study period could have contributed to an increase in influenza-associated mortality, according to the authors. Also, because the models rely on national death certificate data through 2007, preliminary estimates of 2009 influenza A(H1N1)-associated deaths are not comparable, they wrote.

Annual estimates of influenza-associated deaths from 1976-2007 varied substantially by season, influenza virus type, underlying cause of death, and age group, according to revised statistical models, the Centers for Disease Control and Prevention reported August 27 in its Morbidity and Mortality Weekly Report.

The “incredible variation” indicates that using a single, average estimate insufficiently communicates the mortality burden of influenza, Dr. David Shay, medical officer with the CDC’s National Center for Immunization and Respiratory Diseases, said in a media briefing.

With a low of 3,349 estimated deaths in 1986-87 and a high of 48,614 deaths in 2003-2004, the estimated annual rate of influenza-associated deaths in the United States from 1976-2007 ranged from 1.4 to 16.7 deaths per 100,000 persons, according to the new models, which update the CDC’s previously published estimates for 1976-2003 and include new data from 2006-2007.

Because of the wide variability across influenza seasons, “it is relatively meaningless to try to summarize [influenza burden] with one number,” Dr. Shay stressed. “There are very few average seasons, so providing a simple average fails to communicate the impact of flu in an understandable fashion.”

For this reason, the CDC advises quantifying influenza-associated deaths in the context of circulating virus strains and underlying causes of death among age groups. Toward this end, the influenza-associated mortality estimates in the CDC’s revised models are provided for three age groups (younger than 19 years, 19–64 years, and 65 years or older) and for two categories of underlying cause of death codes: pneumonia and influenza causes and respiratory and circulatory causes.

For pneumonia and influenza causes, the respective estimated annual average of influenza-associated deaths and the rate of influenza-associated deaths per 100,000 people were 6,309 and 2.4 for the U.S. population overall; 97 and 0.1 for persons younger than 19 years; 666 and 0.4 for adults age 19-64 years; and 5,546 and 17.0 for adults 65 years and older, the report states.

For deaths with underlying respiratory and circulatory causes, the respective estimated number and rate of influenza-associated deaths per 100,000 was 23,607 and 9.0 for the U.S. population overall; 124 and 0.2 for persons younger than age 19 years; 2,385 and 1.5 among adults age 19–64 years; and 21,098 and 66.1 among adults age 65 years and older (MMWR 2010;33:1057-62).

For both causes, “the average mortality rates for the 22 seasons during which influenza A(H3N2) was a prominent strain were 2.7 times higher than for the nine seasons that it was not,” the authors reported. “The average annual number of influenza-associated deaths during influenza A(H3N2) prominent seasons was 7,722 for pneumonia and influenza causes and 28,909 for respiratory and circulatory causes, compared with 2,856 deaths for pneumonia and influenza causes and 10,648 deaths for respiratory and circulatory causes in seasons in which it was not.”

The numbers “confirm that influenza has a substantial burden of mortality each year, but also that the burden can change substantially from year to year,” Dr. Shay stated.

The findings represented in the revised model are limited by a number of factors, including the failure to account for co-circulating pathogens such as respiratory syncytial virus; the possibility that changing virus surveillance data may reduce the relevance of comparing estimates over time; and the possibility that the increase in the number of adults older than age 65 years during the study period could have contributed to an increase in influenza-associated mortality, according to the authors. Also, because the models rely on national death certificate data through 2007, preliminary estimates of 2009 influenza A(H1N1)-associated deaths are not comparable, they wrote.

Annual estimates of influenza-associated deaths from 1976-2007 varied substantially by season, influenza virus type, underlying cause of death, and age group, according to revised statistical models, the Centers for Disease Control and Prevention reported August 27 in its Morbidity and Mortality Weekly Report.

The “incredible variation” indicates that using a single, average estimate insufficiently communicates the mortality burden of influenza, Dr. David Shay, medical officer with the CDC’s National Center for Immunization and Respiratory Diseases, said in a media briefing.

With a low of 3,349 estimated deaths in 1986-87 and a high of 48,614 deaths in 2003-2004, the estimated annual rate of influenza-associated deaths in the United States from 1976-2007 ranged from 1.4 to 16.7 deaths per 100,000 persons, according to the new models, which update the CDC’s previously published estimates for 1976-2003 and include new data from 2006-2007.

Because of the wide variability across influenza seasons, “it is relatively meaningless to try to summarize [influenza burden] with one number,” Dr. Shay stressed. “There are very few average seasons, so providing a simple average fails to communicate the impact of flu in an understandable fashion.”

For this reason, the CDC advises quantifying influenza-associated deaths in the context of circulating virus strains and underlying causes of death among age groups. Toward this end, the influenza-associated mortality estimates in the CDC’s revised models are provided for three age groups (younger than 19 years, 19–64 years, and 65 years or older) and for two categories of underlying cause of death codes: pneumonia and influenza causes and respiratory and circulatory causes.

For pneumonia and influenza causes, the respective estimated annual average of influenza-associated deaths and the rate of influenza-associated deaths per 100,000 people were 6,309 and 2.4 for the U.S. population overall; 97 and 0.1 for persons younger than 19 years; 666 and 0.4 for adults age 19-64 years; and 5,546 and 17.0 for adults 65 years and older, the report states.

For deaths with underlying respiratory and circulatory causes, the respective estimated number and rate of influenza-associated deaths per 100,000 was 23,607 and 9.0 for the U.S. population overall; 124 and 0.2 for persons younger than age 19 years; 2,385 and 1.5 among adults age 19–64 years; and 21,098 and 66.1 among adults age 65 years and older (MMWR 2010;33:1057-62).

For both causes, “the average mortality rates for the 22 seasons during which influenza A(H3N2) was a prominent strain were 2.7 times higher than for the nine seasons that it was not,” the authors reported. “The average annual number of influenza-associated deaths during influenza A(H3N2) prominent seasons was 7,722 for pneumonia and influenza causes and 28,909 for respiratory and circulatory causes, compared with 2,856 deaths for pneumonia and influenza causes and 10,648 deaths for respiratory and circulatory causes in seasons in which it was not.”

The numbers “confirm that influenza has a substantial burden of mortality each year, but also that the burden can change substantially from year to year,” Dr. Shay stated.

The findings represented in the revised model are limited by a number of factors, including the failure to account for co-circulating pathogens such as respiratory syncytial virus; the possibility that changing virus surveillance data may reduce the relevance of comparing estimates over time; and the possibility that the increase in the number of adults older than age 65 years during the study period could have contributed to an increase in influenza-associated mortality, according to the authors. Also, because the models rely on national death certificate data through 2007, preliminary estimates of 2009 influenza A(H1N1)-associated deaths are not comparable, they wrote.