Homeland Security

Flu Pandemic Morbidity / Mortality

Pandemic years are associated with many more cases of influenza and a higher case fatality rate than that seen in seasonal flu outbreaks. It is common to encounter clinical attack rate ranges for seasonal flu of 5% to 15% in the literature. For pandemic flu, clinical attack rates are reported in the range of 25% to 50%.

During a typical year in the United States, 30,000 to 50,000 persons die as a result of influenza viral infection. Frequently cited numbers are 20,000 deaths each year, and 37,000 annual deaths. About 5-10% of hospitalizations for influenza lead to fatal outcome in adults.

In normal years, although most influenza infection is in children, the serious morbidity and mortality is almost entirely among elderly people with underlying chronic disease. During influenza epidemics from 1979-80 through 2000-01, the estimated overall number of influenza-associated hospitalizations in the United States ranged from approximately 54,000 to 430,000/epidemic. An average of approximately 226,000 influenza-related excess hospitalizations occurred per year, with 63% of all hospitalizations occurring among persons aged > 65 years.

Influenza-related deaths can result from pneumonia and from exacerbations of cardiopulmonary conditions and other chronic diseases. Deaths of older adults account for > 90% of deaths attributed to pneumonia and influenza. In one study of influenza epidemics, approximately 19,000 influenza-associated pulmonary and circulatory deaths per influenza season occurred during 1976-1990, compared with approximately 36,000 deaths during 1990--1999. Estimated rates of influenza-associated pulmonary and circulatory deaths/100,000 persons were 0.4--0.6 among persons aged 0-49 years, 7.5 among persons aged 50--64 years, and 98.3 among persons aged > 65 years.

A different pattern may emerge in a pandemic. The 1918-19 pandemic affected mainly healthy young adults and seemed to spare those at the extremes of life. In the USA, the mortality rate during the 1918 pandemic pandemic was around 2.5%. Similarly, in 1957, the brunt fell on schoolchildren and young adults.

The number of hospitalizations and deaths will depend on the virulence of the pandemic virus. Estimates differ about 10-fold between more and less severe scenarios. Published estimates based on extrapolation of the 1957 and 1968 pandemics suggest that there could be 839,000 to 9,625,000 hospitalizations, 18-42 million outpatient visits, and 20-47 million additional illnesses, depending on the attack rate of infection during the pandemic. Estimates based on extrapolation from the more severe 1918 pandemic suggest that substantially more hospitalizations and deaths could occur. The demand for inpatient and intensive-care unit (ICU) beds and assisted ventilation services could increase by more than 25% under the less severe scenario.

Because the virulence of the influenza virus that causes the next pandemic cannot be predicted, two scenarios were presented by CDC, HHS and DHS based on extrapolation of past pandemic experience. The DHS estimates are suspect, since they appear to derive from a 1999 analysis that was based on the 1997 US population of 265 million. By 2005 the US population was about 295 million, so the DHS estimates are about 10% low simply due to the growth in population.

According to the Centers for Disease Control and Prevention (CDC), it has been estimated that in the absence of any control measures such as vaccination and drugs, a "medium-level" influenza pandemic in the United States could kill 89,000 to 207,000 people, affect from 15 to 35 percent of the U.S. population, and generate associated costs ranging from $71 billion to $167 billion. Another Centers for Disease Control and Prevention (CDC) estimate suggested that, in the United States alone, up to 200 million people will be infected, 50 million people will require outpatient care, two million people will be hospitalized, and between 100,000 and 500,000 persons will die. These numbers are significantly higher than the estimates used by the Deparment of Homeland Security. The HHS notes that the death rate associated with the 1918 influenza applied to the current population would produce 1.9 million deaths in the United States and 180 million to 360 million deaths globally. It is most noteworthy that the "Low" scenario presented by HHS corresponds to the "High" scenario presented by DHS.

By 2005 the observed human avian flu case fatality rate declined to 34% (16/47) in northern Viet Nam, but was 83.3% (20/24) in southern Viet Nam. The case fatality rate in Thailand was 71% (12/17) and 100% in Cambodia (4/4) in 2004. The case fatality rate was 89% among those younger than 15 years of age in Thailand. Death occurred an average of 9 or 10 days after the onset of illness (range, 6 to 30) and most patients have died of progressive respiratory failure.

Avian Influenza (H5N1) viruses isolated from humans in Asia in 2004 exhibited increased virulence in laboratory test mammals compared to the viruses isolated from 1997 human cases. By one authoritative estimate, an H5N1 avian influenza that is transmittable from human to human could be devastating: assuming a mortality rate of 20 percent and 80 million illnesses, the United States could suffer 16 million deaths.

Undetected cases might imply that infections with H5N1 influenza may be more common than previously thought, suggesting that the overall case fatality rate may not be as high as previously suggested. It also raises the question of whether mild and/or asymptomatic cases of avian flu allow the virus more opportunities to mix, or "re-assort," with human-adapted flu viruses. This genetic mixing increases the likelihood of generating a virus that is able to efficiently spread from person to person.

HHS Health Outcomes
Characteristic Moderate (1958/68-like) Severe (1918-like)
Illness 90 million (30%) 90 million (30%)
Outpatient medical care 45 million (50%) 45 million (50%)
Hospitalization 865,000 9,900,000
ICU care 128,750 1,485,000
Mechanical ventilation 64,875 742,500
Deaths 209,000 1,903,000

Mean DHS estimates (5th, 95th percentiles) of the impact of the next influenza pandemic in the United States without any large-scale and/or effective interventions

DHS Health Outcomes 15% Gross Attack Rate*
(5th, 95th percentiles)
35% Gross Attack Rate
(5th, 95th percentiles)
Fatalities 87,000
(54,400; 122,200)
(127,200; 285,300)
Hospitalizations 314,400
(210,400; 417,200)
(491,000; 973,500)
Outpatient visits 18.1 million
(17.5; 18.7)
42.2 million
(40.8; 43.7)
Self-care ill 21.3 million
(20.6; 21.9)
49.7 million
(48.2; 51.2)
*Percent Gross Attack Rate refers to the percentage of the entire U.S. population that will have a clinical case of influenza.

Based on the DHS estimates, the economic impact, in 2004 US dollars, would range from $87 billion (15% gross attack rate) to $203 billion (35% gross attack rate). These estimates include a value for time lost from work but do not include any estimate due to economic disruption or long-term health care costs.

Property damage is minimal. Service disruption, however, could be severe due to worker illness. Health care systems will be severely stressed, if not overwhelmed, and first responders are also likely to be severely strained.

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