Influenza (Flu)
Influenza is an acute, self-limiting disease caused by type A and B influenza viruses, characterized by high fever and seasonal outbreaks of varying intensity every winter.
Influenza differs from other upper respiratory tract infections in three key ways:
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It causes epidemics, rapidly spreading among populations.
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It presents with significantly high fever, unlike other respiratory infections.
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It can lead to pneumonia as a complication, increasing mortality rates.
Understanding influenza epidemiology is essential to assess vaccine effectiveness. Each year, influenza has a unique attack rate ranging from 10% to 30% globally, with new variants sometimes leading to pandemics.
Pandemics and Epidemics
Influenza pandemics typically originate in a single geographic region and spread rapidly via travel networks. They affect all age groups, though fatality rates do not necessarily increase.
When a pandemic starts, the infection spreads rapidly, peaking in three weeks and resolving within eight weeks. Infection rates tend to be higher in infants and children than in adults, though hospitalization rates remain higher among adults. Families with school-age children experience the highest infection rates.
The scale of each epidemic or pandemic is determined by:
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The degree of antigenic variation in the new virus.
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The virulence of the virus strain.
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Existing immunity levels in the population.
During a typical North American epidemic, attack rates range between 10%–20% in large populations and 40%–50%in specific groups such as schoolchildren or nursing home residents.
Hospitalization rates are highest among:
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Children aged 0–4 years (1,000 per 100,000 for healthy children, 500 per 100,000 for high-risk groups).
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Adults over 65 years (200 per 100,000 for healthy individuals, over 1,000 per 100,000 for high-risk groups).
