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Influenza and Pandemic Influenza: A Primer
Dr. Orenstein is Professor of Medicine and Pediatrics, Emory University School of Medicine, and was formerly Director, National Immunization Program of the Centers for Disease Control and Prevention (CDC).
Dr. Orenstein has been a consultant to Chiron, has received research funds from the Chiron Foundation and has previously received research funds from Medimmune. Few infectious disease burdens can compare with the impact of global influenza pandemics. During the past century, the United States was affected by three pandemics, 1918-19, 1957-58, and 1968-69 which accounted for approximately 500,000, 70,000, and 34,000 deaths, respectively.(1) In addition, pandemic influenza can be associated with severe strains on the nation's health care delivery capacity as well as substantial economic disruptions. The Department of Health and Human Services has estimated (Table 1) the health burden of a moderate and a severe pandemic in the United States.(2) Table 1.
Number of Episodes of Illness, Healthcare Utilization and Death Associated with Moderate and Severe Pandemic Influenza Scenarios.*
*Estimates based on extrapolation from past pandemics in the United States. Note that these estimates do not include the potential impact of interventions not available during the 20th century pandemics.
Source: www.pandemicflu.gov, p18. Overall, approximately 30% of the population would develop clinical disease, approximately 50% of these persons would seek medical care and up to 1.9 million would die of their illness. Reports of human cases of an avian influenza virus, H5N1, in at least seven countries since 2003, raise concerns that a new pandemic may be at hand.(3) While most influenza experts believe a pandemic of influenza is inevitable, it is not clear when the pandemic will strike, which virus will be its cause and how severe the health burden will be. However, it is important to enhance our preparedness now to be ready for the next pandemic, whenever it should occur. The Influenza Virus
The influenza virus consists of eight segmented genes.(4)
Figure 1.
Influenza Virus.
Cartoon of the influenza virus with its eight segmented genes and two critical surface glycoproteins, the hemagglutinin (HA) and the neuraminidase (NA).
There are two sugar-based proteins (glycoproteins), the hemagglutinin (H) and the neuraminidase (N), which project from the virus surface and play a critical role in the disease process. The hemagglutinin attaches to receptors on the host cell surface thus allowing the virus to enter5 The neuraminidase is important in final packaging and release of virus from infected cells and may be important in disease severity. There are two major types of viruses associated with human disease, influenza A and influenza B.(4) Influenza B viruses are stable, that is, their mutations are usually small, they don't mutate into completely new versions, and are not associated with pandemics. Influenza B viruses tend to cause epidemics of influenza which vary in magnitude, severity and interval in years from prior epidemics. Influenza A viruses are associated with both annual epidemics and global pandemics. There are 16 known hemagglutinins of the influenza A virus, H1 - H16 and nine neuraminidases, N1-N9. Water fowl, particularly ducks and geese, are the natural hosts for the virus.(6) Only three hemagglutinins are known to have circulated in humans — H1, H2 and H3. The H1N1 virus circulated from 1918-19 until 1957 when it was replaced by the A/Asian, H2N2 virus. This virus continued to circulate until 1968 when an H3N2 virus became predominant. In 1977, an H1 virus returned, which was similar to earlier strains of H1N1. This latter strain primarily affects young persons since persons born prior to 1957 were likely exposed to a similar virus and are usually immune. At the present time, three types of influenza viruses circulate in humans, A/H3N2 which tends to be responsible for the most severe of the annual epidemics, A/H1N1 and B. The strains that predominate in a given year vary.
(1) Comment has been made alexd180
(USA)
Very nice site!Posted Mon, May. 24, 2010 at 7:35 am EDT
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