Types of Influenza Viruses
There are four types of influenza viruses: A, B, C and D. Human influenza A and
B viruses cause seasonal epidemics of disease almost every winter in the United
States. The emergence of a new and very different influenza A virus to infect
people can cause an influenza pandemic. Influenza type C infections generally
cause a mild respiratory illness and are not thought to cause epidemics.
Influenza D viruses primarily affect cattle and are not known to infect or cause
illness in people.
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
18 different hemagglutinin subtypes and 11 different neuraminidase subtypes. (H1
through H18 and N1 through N11 respectively.)
Influenza A viruses can be further broken down into different strains. Current
subtypes of influenza A viruses found in people are influenza A (H1N1) and
influenza A (H3N2) viruses. In the spring of 2009, a new influenza A (H1N1)
virus (CDC
2009 H1N1 Flu website) emerged to cause illness in people. This virus
was very different from the human influenza A (H1N1) viruses circulating at that
time. The new virus caused the first influenza pandemic in more than 40 years.
That virus (often called “2009 H1N1”) has now replaced the H1N1 virus that was
previously circulating in humans.
Influenza B viruses are not divided into subtypes, but can be further broken
down into lineages and strains. Currently circulating influenza B viruses belong
to one of two lineages: B/Yamagata and B/Victoria.
CDC follows an internationally accepted naming convention for influenza viruses.
This convention was accepted by WHO in 1979 and published in February 1980 in
the Bulletin of the World Health Organization, 58(4):585-591 (1980) (see
A revision of the system of nomenclature for
influenza viruses: a WHO Memorandum[854 KB, 7 pages]). The approach
uses the following components:
·
The antigenic type (e.g., A, B, C)
·
The host of origin (e.g., swine, equine, chicken, etc. For human-origin viruses,
no host of origin designation is given.)
·
Geographical origin (e.g., Denver, Taiwan, etc.)
·
Strain number (e.g., 15, 7, etc.)
·
Year of isolation (e.g., 57, 2009, etc.)
·
For influenza A viruses, the hemagglutinin and neuraminidase antigen description
in parentheses (e.g., (H1N1), (H5N1)
For example:
·
A/duck/Alberta/35/76 (H1N1) for a virus from duck origin
·
A/Perth/16/2009 (H3N2) for a virus from human origin
Influenza A (H1N1), A (H3N2), and one or two influenza B viruses (depending on
the vaccine) are included in each year’s influenza vaccine. Getting a flu
vaccine can protect against flu viruses that are the same or related to the
viruses in the vaccine. Information about this season’s vaccine can be found at
Preventing Seasonal Flu with Vaccination.
The seasonal flu vaccine does not protect against influenza C viruses.
Additionally, flu vaccines will NOT protect against infection and illness caused
by other viruses that also can cause influenza-like symptoms. There are many
other non-flu viruses that can result in influenza-like illness (ILI) that
spread during flu season.