Epidemiology of influenza
Section snippets
Seasonality of influenza
The fact that, in the temperate zone, influenza is highly seasonal is a major reason that it has been possible to estimate the impact of influenza without identifying those infected using laboratory techniques. In the Northern Hemisphere, there may be viruses identified at other times of the year, but outbreaks rarely begin before late November. Generally, outbreaks dominated by A (H3N2) start earlier, with type B following and often closing the season [13], [14]. Especially in those years in
Effect of influenza type and subtype and of age
The two type A viruses (H3N2) and (H1N1) and type B viruses produce illnesses of relatively similar clinical characteristics. However, in older children and healthy adults, it is possible to demonstrate that A (H3N2) virus is associated with the most severe illnesses, A (H1N1) the mildest, and type B intermediate. This can be observed in differences in overall duration of uncomplicated illnesses in the general population [12]. In the elderly and in other at-risk populations, complications
Influenza impact: mortality and hospitalizations
The historic observations that, in influenza outbreaks, certain population groups were at increased risk of death were made at a time when the influenza virus had not been identified. Even when influenza viruses could be isolated, identification of the viral etiology of an event as rare as death was not feasible, especially with the techniques then available. Several epidemiologic methods have been developed to estimate the number of deaths associated with influenza, using laboratory data as a
Influenza morbidity and indirect protection
It is much more difficult to quantify influenza morbidity. While it is possible to identify typical influenza based on clinical characteristics during the influenza season, much influenza is milder and would not be recognized without laboratory confirmation. Such studies, which have been done in large enough populations and over several seasons, characteristics which allow generalizability have been found [38]. A generalization, based on a limited number of studies conducted long enough, is
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