The Three Types of Influenza Viruses
Influenza A, B, and C
Influenza types A or B viruses cause epidemics of disease almost every winter.
In the United States, these winter influenza epidemics can cause illness in
10% to 20% of people and are associated with an average of 36,000 deaths and
114,000 hospitalizations per year. Getting a flu shot can prevent illness from
types A and B influenza. Influenza type C infections cause a mild respiratory
illness and are not thought to cause epidemics. The flu shot does not protect
against type C influenza.
Influenza type A viruses are divided into subtypes based on two proteins on
the surface of the virus. These proteins are called hemagglutinin (H) and neuraminidase
(N). The current subtypes of influenza A viruses found in people are A(H1N1)
and A(H3N2). Influenza B virus is not divided into subtypes. Influenza A(H1N1),
A(H3N2), and influenza B strains are included in each year's influenza vaccine.
Transmission of Influenza Viruses from Animals to People
Influenza A viruses are found in many different animals, including ducks, chickens,
pigs, whales, horses, and seals. Influenza B viruses circulate widely only among
humans.
Influenza A viruses are divided into subtypes based on two proteins on the
surface of the virus: the hemagglutinin (H) and the neuraminidase (N). There
are 15 different hemagglutinin subtypes and 9 different neuraminidase subtypes,
all of which have been found among influenza A viruses in wild birds. Wild birds
are the primary natural reservoir for all subtypes of influenza A viruses and
are thought to be the source of influenza A viruses in all other animals. Most
influenza viruses cause asymptomatic or mild infection in birds; however, the
range of symptoms in birds varies greatly depending on the strain of virus.
Infection with certain avian influenza A viruses (for example, some strains
of H5 and H7 viruses) can cause widespread disease and death among some species
of wild and especially domestic birds such as chickens and turkeys.
Pigs can be infected with both human and avian influenza viruses in addition
to swine influenza viruses. Infected pigs get symptoms similar to humans, such
as cough, fever, and runny nose. Because pigs are susceptible to avian, human
and swine influenza viruses, they potentially may be infected with influenza
viruses from different species (e.g., ducks and humans) at the same time. If
this happens, it is possible for the genes of these viruses to mix and create
a new virus. For example, if a pig were infected with a human influenza virus
and an avian influenza virus at the same time, the viruses could mix (reassort)
and produce a new virus that had most of the genes from the human virus, but
a hemagglutinin and/or neuraminidase from the avian virus. The resulting new
virus would likely be able to infect humans and spread from person to person,
but it would have surface proteins (hemagglutinin and/or neuraminidase) not
previously seen in influenza viruses that infect humans. This type of major
change in the influenza A viruses is known as antigenic shift. Antigenic shift
results when a new influenza A subtype to which most people have little or no
immune protection infects humans. If this new virus causes illness in people
and can be transmitted easily from person to person, an influenza pandemic can
occur.
While it is unusual for people to get influenza infections directly from animals,
sporadic human infections and outbreaks caused by certain avian influenza A
viruses have been reported. For more information see: Avian Influenza Infections
in Humans.
How the Flu Virus Can Change - "Drift" and "Shift"
Influenza viruses can change in two different ways.
One is called "antigenic drift." These are small changes in the virus
that happen continually over time. Antigenic drift produces new virus strains
that may not be recognized by the body's immune system. This process works as
follows: a person infected with a particular flu virus strain develops antibody
against that virus. As newer virus strains appear, the antibodies against the
older strains no longer recognize the "newer" virus, and reinfection
can occur. This is one of the main reasons why people can get the flu more than
one time. In most years, one or two of the three virus strains in the influenza
vaccine are updated to keep up with the changes in the circulating flu viruses.
So, people who want to be protected from flu need to get a flu shot every year.
The other type of change is called "antigenic shift." Antigenic shift
is an abrupt, major change in the influenza A viruses, resulting in new hemagglutinin
and/or new hemagglutinin and neuraminidase proteins in influenza viruses that
infect humans. Shift results in a new influenza A subtype. When shift happens,
most people have little or no protection against the new virus. While influenza
viruses are changing by antigenic drift all the time, antigenic shift happens
only occasionally. Type A viruses undergo both kinds of changes; influenza type
B viruses change only by the more gradual process of antigenic drift.
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