Influenza in the Elderly
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INTRODUCTION
Influenza is a very common respiratory illness. It usually occurs in 15-25% of the world’s human population each year.1-3 This illness typically has a low death rate, with average overall mortality rates of 0.1% or less. However, influenza carries much higher rates of morbidity and mortality in the elderly and other vulnerable populations,2-5 and thus it is of special interest to geriatricians and public health physicians.
Influenza epidemics have been well documented since Buonissequi described the “grande influenza” in 1357.6 At least 31 influenza pandemics have occurred since 1580.6 One strain of influenza A, the so-called “Spanish flu” of 1918-1919, was the most deadly infectious disease epidemic in recorded history, resulting in perhaps 50 million deaths worldwide, and killing 8-10% of all young persons living at that time.7,8 While the 1918 pandemic killed young and old alike, more recent outbreaks have proven fatal for the elderly and debilitated, but rarely for young, healthy patients.
In this article, we review the epidemiology, clinical features, complications, and treatment of influenza A in the elderly. Yearly vaccination of the elderly is an important strategy to prevent influenza and lessen its morbidity in these patients. Other strategies include vaccination of health care workers and caregivers, universal childhood influenza A vaccination, early recognition and treatment of influenza A in the elderly, and use of respiratory hygiene in health care settings. The recent emergence of the H5N1 avian strain of influenza A has raised concerns about a worldwide pandemic, and has resulted in planning at national and international levels.
THE VIRUS AND HUMAN INFECTION
Influenza is a medium-sized RNA virus of the Orthomyxoviridae family. There are three main types: A, B, and C. Influenza virus B and C infect only humans and tend to cause mild upper respiratory illnesses. Influenza A infects birds, horses, and swine, and can cause severe illness and death in vulnerable human populations. Influenza A will be the subject of subsequent discussion in this article.
The segmented RNA of influenza A virus codes for four major proteins, two of which project from the surface of the virus: hemagglutinin (HA), which is the site of viral attachment to host cells, and neuraminidase (NA), which splits neuraminic acid from mucoprotein and may be involved in viral particle assembly. These surface proteins are highly antigenic. Two to four weeks after infection, patients develop antibodies to both HA and NA.3 Antibodies against HA provide the best protection against subsequent infection.6 Influenza A is especially facile in being able to alter these surface proteins. This antigenic variation is responsible for the virus’s ability to cause illness in a significant proportion of the population each year.
Minor variations in the HA or NA proteins is termed antigenic drift. Some persons who were previously infected with similar strains may be protected against infection by influenza A viral strains demonstrating these minor variations, but a significant minority can be infected with the new strain. Periodically, influenza A virus will emerge with major changes in the HA or NA polypeptides. This antigenic shift results in a novel strain of the virus, to which very few persons in the population have antigenic protection. When this happens, a pandemic occurs, and a majority of the world’s population will be infected with the virus that year. Before 1957, H1N1 types of influenza A circulated and demonstrated antigenic drift each year.9 The “Asian flu” pandemic of 1957 was the result of an H2N2 strain. Varieties of H2N2 strains then circulated until they were replaced with H3N2 strains that emerged after the “Hong Kong flu” pandemic of 1968. Recently, several strains have circulated simultaneously.9
In infected humans, influenza A replicates in the respiratory epithelium, causing cell death.
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