Key Facts About Avian Influenza A
What is an avian influenza A (H5N1) virus?
Influenza A (H5N1) virus – also called "H5N1 virus" – is an influenza A virus
subtype that occurs mainly in birds. It was first isolated from birds (terns) in
South Africa in 1961. Like all bird flu viruses, H5N1 virus circulates among
birds worldwide, is very contagious among birds, and can be deadly.
What is the H5N1 bird flu that has recently been reported in Asia?
Outbreaks of influenza H5N1 occurred among poultry in eight countries in Asia
(Cambodia, China, Indonesia, Japan, Laos, South Korea, Thailand, and Vietnam)
during late 2003 and early 2004. At that time, more than 100 million birds in
the affected countries either died from the disease or were killed in order to
try to control the outbreak. By March 2004, the outbreak was reported to be
under control. Beginning in late June 2004, however, new deadly outbreaks of
influenza H5N1 among poultry were reported by several countries in Asia
(Cambodia, China, Indonesia, Malaysia [first-time reports], Thailand, and
Vietnam). It is believed that these outbreaks are ongoing. Human infections of
influenza A (H5N1) have been reported in Thailand, Vietnam and Cambodia.
What is the risk to humans from the H5N1 virus in Asia?
The H5N1 virus does not usually infect humans. In 1997, however, the first
case of spread from a bird to a human was seen during an outbreak of bird flu in
poultry in Hong Kong. The virus caused severe respiratory illness in 18 people,
6 of whom died. Since that time, there have been other cases of H5N1 infection
among humans. Most recently, human cases of H5N1 infection have occurred in
Thailand, Vietnam and Cambodia during large H5N1 outbreaks in poultry. The death
rate for these reported cases has been about 50 percent. Most of these cases
occurred from contact with infected poultry or contaminated surfaces; however,
it is thought that a few cases of human-to-human spread of H5N1 have occurred.
So far, spread of H5N1 virus from person to person has been rare and spread
has not continued beyond one person. However, because all influenza viruses have
the ability to change, scientists are concerned that the H5N1 virus could one
day be able to infect humans and spread easily from one person to another.
Because these viruses do not commonly infect humans, there is little or no
immune protection against them in the human population. If the H5N1 virus were
able to infect people and spread easily from person to person, an
"influenza pandemic" (worldwide outbreak
of disease) could begin. No one can predict when a pandemic might occur.
However, experts from around the world are watching the H5N1 situation in Asia
very closely and are preparing for the possibility that the virus may begin to
spread more easily and widely from person to person.
How is infection with H5N1 virus in humans treated?
The H5N1 virus currently infecting birds in Asia that has caused human
illness and death is resistant to amantadine and rimantadine, two antiviral
medications commonly used for influenza. Two other antiviral medications,
oseltamavir and zanamavir, would probably work to treat flu caused by the H5N1
virus, though studies still need to be done to prove that they work.
Is there a vaccine to protect humans from H5N1 virus?
There currently is no vaccine to protect humans against the H5N1 virus that
is being seen in Asia. However, vaccine development efforts are under way.
Research studies to test a vaccine to protect humans against H5N1 virus began in
April 2005. (Researchers are also working on a vaccine against H9N2, another
bird flu virus subtype.) For more information about the H5N1 vaccine development
process, visit the
National Institutes of Health website.
What is the risk to people in the United States from the H5N1 bird flu
outbreak in Asia?
The current risk to Americans from the H5N1 bird flu outbreak in Asia is low.
The strain of H5N1 virus found in Asia has not been found in the United States.
There have been no human cases of H5N1 flu in the United States. It is possible
that travelers returning from affected countries in Asia could be infected.
Since February 2004, medical and public health personnel have been watching
closely to find any such cases.
What does CDC recommend regarding the H5N1 bird flu outbreak in Asia?
In February 2004, CDC provided U.S. health departments with recommendations
for enhanced surveillance ("detection") in the U.S. of avian influenza A (H5N1).
Follow-up messages (Health Alert Network) were sent to the health departments on
August 12, 2004, and February 4, 2005, both reminding health departments about
how to detect (domestic surveillance), diagnose, and prevent the spread of avian
influenza A (H5N1). It also recommended measures for laboratory testing for H5N1
virus. CDC currently advises that travelers to countries in Asia with known
outbreaks of influenza A (H5N1) avoid poultry farms, contact with animals in
live food markets, and any surfaces that appear to be contaminated with feces
from poultry or other animals.
What is CDC doing to prepare for a possible H5N1 flu pandemic?
CDC is taking part in a number of pandemic prevention and preparedness
- Working with the Association of Public Health Laboratories on training
workshops for state laboratories on the use of special laboratory (molecular)
techniques to identify H5 viruses.
- Working with the Council of State and Territorial Epidemiologists and
others to help states with their pandemic planning efforts.
- Working with other agencies such as the Department of Defense and the
Veterans Administration on antiviral stockpile issues.
- Working with the World Health Organization (WHO) and Vietnamese Ministry
of Health to investigate influenza H5N1 in Vietnam and to provide help in
laboratory diagnostics and training to local authorities.
- Performing laboratory testing of H5N1 viruses.
- Starting a $5.5 million initiative to improve influenza surveillance in
- Holding or taking part in training sessions to improve local capacities to
conduct surveillance for possible human cases of H5N1 and to detect influenza
A H5 viruses by using laboratory techniques.
- Developing and distributing reagents kits to detect the currently
circulating influenza A H5N1 viruses.
Working together with WHO and the National Institutes of Health (NIH) on
safety testing of vaccine seed candidates and to develop additional vaccine
virus seed candidates for influenza A (H5N1) and other subtypes of influenza A
virus. For more information please go to