Overview of Influenza
Influenza (flu) A & B are members of the virus family
Orthomyxoviridae and are distinguished by the proteins that make up their viral
coatings. Flu A subtypes include the 2009 H1N1 swine flu and H5N1 bird flu.
While both influenza A and B are capable of causing disease
in humans, flu A is more common (71% of diagnosed cases)1, more virulent,
and responsible for most of the flu pandemics2:
- Flu A has animal as well as human hosts
- Flu A mutates too rapidly for the immune system to
create antibodies against current strains3
- Flu B has only a human host
- Flu B mutates slowly and humans have the time to
The yearly trivalent flu vaccine protects against two
strains of influenza A and one strain of influenza B.
Flu subspecies are defined by the proteins on their coating.
The “H” stands for hemagglutinin, the “N” for the neuraminidase. There are 16 versions
of hemagglutinin and 9 versions of neuraminidase. For example, 2009 H1N1 flu, contains
hemagglutinin 1 and neuraminidase 1.
Once flu virus invades the airway, there is an incubation
period of 18-72 hours. The virus causes cellular dysfunction and degeneration as
it reproduces. Most patients recover from seasonal flu without long-term effects.
However, various versions of flu A are capable of causing serious disease and death
even in otherwise healthy persons.5
Because of viral shedding, adults infected with flu can
be contagious 1 day before symptoms begin and for 5-7 days after the onset of illness.
Children can remain contagious for even longer.6
Epidemiology of influenza
Influenza is one of the most common infections worldwide.
According to the Centers for Disease Control & Prevention (CDC), on average
each year in the United States7:
Influenza infection follows predictable seasonal patterns
that have remained consistent since the CDC began surveillance. In the U.S., winter
is the season for infection and the peak month for infection is Februrary.8
- 5% to 20% of the population gets the flu
- More than 200,000 people are hospitalized due to
complications of flu
- About 36,000 people die from flu-related causes