H1N1 Update: CDC: Flu's Toll; Seasonal Vaccine Not Effective...

H1N1 Update: Estimates of Flu's Toll; Seasonal Vaccine Not Effective Against 2009 H1N1

Reports on 2009 H1N1 influenza in the U.S. will now use estimates from the CDC's Emerging Infection Program, rather than counting only laboratory-confirmed cases, according to a CDC news briefing.

The new estimates for the first 6 months of the pandemic — from mid-April to mid-October — find that:

• Roughly 22 million people in the U.S. became ill from the virus.
• Nearly 100,000 were hospitalized.
• Some 3900 died, including an estimated 540 children under 18; some 2900 adults between 18 and 64; and 440 elderly.

This week's MMWR carries a CDC analysis concluding that the seasonal trivalent vaccine offers no protection from — or increased risk for — 2009 H1N1 disease. An additional surveillance article on the pandemic notes that "severe outcomes among children ... continue to be prominent" and provide support for the recommendation that those aged 6 months to 24 years be targeted for vaccination.

Also on Thursday, the FDA granted accelerated approval for the use of CSL Limited's 2009 H1N1 influenza vaccine in children 6 months and up. The vaccine had previously been indicated for adults.

CDC Estimates of 2009 H1N1 Cases and Related Hospitalizations and Deaths from April-October 17, 2009, By Age Group

2009 H1N1	Mid-Level Range*	Estimated Range *
Cases
0-17 years	~8 million	~5 million to ~13 million
18-64 years	~12 million	~7 million to ~18 million
65 years and older	~2 million	~1 million to ~3 million
Cases Total	~22 million	~14 million to ~34 million
Hospitalizations
0-17 years	~36,000	~23,000 to ~57,000
18-64 years	~53,000	~34,000 to ~83,000
65 years and older	~9,000	~6,000 to ~14,000
Hospitalizations Total	~98,000	~63,000 to ~153,000
Deaths
0-17 years	~540	~300 to ~800
18-64 years	~2,920	~1,900 to ~4,600
65 years and older	~440	~300 to ~700
Deaths Total	~3,900	~2,500 to ~6,100

* Deaths have been rounded to the nearest ten. Hospitalizations have been rounded to the nearest thousand and cases have been rounded to the nearest million. Exact numbers also are available. 

Under-Counting of Flu-Related Deaths

CDC does not know exactly how many people die from seasonal flu each year. There are several reasons for this:

• First, states are not required to report individual seasonal flu cases or deaths of people older than 18 years of age to CDC. 
  
• Second, seasonal influenza is infrequently listed on death certificates of people who die from flu-related complications.
• Third, many seasonal flu-related deaths occur one or two weeks after a person’s initial infection, either because the person may develop a secondary bacterial co-infection (such as a staph infection) or because seasonal influenza can aggravate an existing chronic illness (such as congestive heart failure or chronic obstructive pulmonary disease).
• Also, most people who die from seasonal flu-related complications are not tested for flu, or they seek medical care later in their illness when seasonal influenza can no longer be detected from respiratory samples. Influenza tests are most likely to detect influenza if performed soon after onset of illness.
• For these reasons, many flu-related deaths may not be recorded on death certificates.

These are some of the reasons that CDC and other public health agencies in the United States and other countries use statistical models to estimate the annual number of seasonal flu-related deaths. (Flu deaths in children were made a nationally notifiable condition in 2004, and since then, states have reported flu-related child deaths in the United States through the Influenza Associated Pediatric Mortality Surveillance System).

CDC paper in Emerging Infectious Diseases: “Estimates of the Prevalence of Pandemic (H1N1) 2009, United States, April–July 2009”