Thursday, August 26, 2010

MMWR: Estimates Of Yearly Seasonal Influenza Deaths



# 4836



One of the downsides to employing averages, rule’s of thumb, and oversimplified but `friendly’ data is that it can skew the public’s perception of what is `normal’ or expected.


A prime example is the oft stated statistic that 36,000 Americans die each year from an influenza-related illness. 


It’s an easy and convenient `factoid’ that the media can slip into a news report, but it isn’t a completely reliable number.


As I’ve mentioned in the past, that number is an estimate . . . not an actual count . . . and it can be a bit misleading because it is an average of estimated deaths that occurred during the 1990s – a number that was calculated as ranging from a low of 17,000 to a high of 52,000.


None of this is hidden knowledge, of course.  The CDC has readily stated how they came up with their estimates, and those methods have been the subject of considerable debate.


While everyone wants good numbers, there are a number of obstacles in the way of any attempt to accurately estimate flu-related deaths.




  • Estimates are extrapolated based on a surveillance subset of the country, not the whole nation
  • There are often co-circulating viruses that may influence overall mortality.
  • Surveillance, testing, and reporting may change over time
  • Different mathematical models can produce differing results
  • There are varying opinions as to what constitutes an influenza-related fatality.


When combined with the inevitable variations in the severity of influenza seasons (H3 years are usually more severe than H1 years), this makes it impossible to derive a single number that `works’.


In an attempt to help us better understand the impact of influenza-related deaths in the United States each year, the CDC has released a new study in today’s MMWR  titled:


Estimates of Deaths Associated with Seasonal Influenza --- United States, 1976--2007


August 27, 2010 / 59(33);1057-1062


There is a lot of data here, as deaths are divided into two broad camps ( 1) pneumonia and influenza causes and 2) respiratory and circulatory causes), and further divided by three age brackets (<19 years, 19--64 years, and ≥65 years)


For deaths with underlying pneumonia and influenza causes (the most narrow definition of flu-related fatalities used) the models estimated a yearly average of 6,309 (range: 961 in 1986--87 to 14,715 in 2003--04) influenza-associated deaths.


Using a broader criteria (underlying respiratory and circulatory causes including pneumonia and influenza causes)  the models estimated an annual average of 23,607 (range: 3,349 in 1986--87 to 48,614 in 2003--04) influenza-associated deaths.


Somewhat less than the old 36,000 number, but explained in the report as that number having been based on estimates derived during the 1990s – when H3N2 was the predominant strain.


These new numbers are based on 31 influenza seasons, running 1976-2007.

The short course  from the study is as follows. 


What is already known on this topic?

Influenza infections result in substantial medical costs, hospitalizations, lost productivity, and thousands of deaths every year in the United States with the majority of deaths from seasonal influenza occurring among adults aged ≥65 years.


What is added by this report?

During 1976--2007, annual estimates of influenza-associated deaths from respiratory and circulatory causes ranged from 3,349 (in 1986--87) to 48,614 (in 2003--04), and the annual rate of influenza-associated death for all ages ranged from 1.4 to 16.7 deaths per 100,000 persons; during seasons when influenza A(H3N2) circulating strains were prominent, 2.7 times more deaths occurred compared with seasons when A(H3N2) was not prominent.


What are the implications for public health practice?

A single estimate should not be used to summarize influenza-associated deaths; a range of estimates should be described in the context of circulating virus strains and underlying causes of death among age groups. Annual influenza vaccination (now recommended for all persons aged ≥6 months) is the best way to avoid influenza complications, and prompt treatment with antiviral medications can reduce the risk for severe illness and death among persons at increased risk for influenza or who are hospitalized with influenza.



Since statistical analysis isn’t my strong suit, I’ll leave it to others to try to analyze these findings and methods.

By just about any calculation, however, influenza is a serious illness that exacts a heavy toll on society.  Which makes getting the yearly flu vaccine a smart move.


By all means, follow this link to read the report in its entirety.

No comments: