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reducing pandemic influenza virus transmission. Table 1 provides a summary of the committee’s conclusions regarding the community interventions and Table 2 provides a listing of the recommendations. These tables can be found at the end of the report.

INFLUENZA EPIDEMIOLOGY

Influenza, an infectious disease that causes an estimated 36,000 or more deaths in the United States during a typical influenza season, has a clinical attack rate that is highest in young children but a case-fatality rate that is highest in the elderly. One measure of infectivity is “R0”, the average number of secondary cases of disease generated by a typical primary case in a susceptible population. Influenza has an R0 that typically ranges from 1.5–3.1 The United States’ vaccination strategy has long been geared to decreasing individual risk, rather than community transmission, by focusing on the elderly and those with chronic health care conditions that increase the risk for severe illness, hospitalization, or death. Recently, the recommendations for vaccination have expanded to include young children and people over 50 years of age. The incubation period for seasonal influenza is approximately 2 days (range is 1–4 days) and is most communicable beginning 1 day before onset of symptoms and up to five days thereafter. Despite the cumulative toll of influenza in the United States and the rest of the world, there remain key unknowns that are relevant to discussions of pandemic influenza. A significant unknown relates to the mode of transmission of influenza; namely, is the virus is primarily transmitted through droplets, aerosol, or contact with fomites.2 This uncertainty is significant because it calls into questions some key “tried and true” interventions that are used for protecting against seasonal influenza, as will be described in subsequent sections. There are also unknowns about the virus itself—particularly what changes in the virus are predictive of infectivity, case-fatality, and responsiveness to antiviral drugs. All these uncertainties are magnified when considering pandemic influenza.

Three previous pandemics occurred during the 20th century.3 The 1918-1919 pandemic (often referred to as the “Spanish influenza”) was associated with 500,000 deaths in the United States and over 20 million (and possibly up to 100 million) deaths worldwide. The subsequent pandemics were milder. The 1957 “Asian influenza” was associated with 69,800 deaths in the US and the 1958 “Hong Kong influenza” with approximately 33,800 deaths in the US. The 1918-1919 pandemic was unusual in that significant mortality occurred in young, healthy adults, in addition to groups usually affected by influenza, such as infants, the elderly, and the ill.

As has been said many times, a pandemic of influenza is “long overdue”. There is little doubt that the world will experience another pandemic, but there are many uncertainties about this pandemic. For instance, no one knows when the pandemic will occur. It could arrive soon, emerging from mutations and/or reassortment of the currently worrisome H5N1 virus circulating in wildlife in Asia, eastern Europe, and Africa. The currently circulating H5N1 virus could also remain primarily a virus of wildlife and poultry and never become a significant human pathogen. The next pandemic virus might not be

1

Measles, for example, is much more infectious and has an R0 of approximately 10.

2

An inanimate object that can transmit infectious agents from one person to another through contact or touching.

3

See http://www.hhs.gov/nvpo/pandemics/flu3.htm for more information.



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