Archived: Highly Pathogenic Asian Avian Influenza A(H5N1) in People

Sporadic Cases of Asian H5N1 Have Occurred in People

More than 700 human infections with Asian HPAI H5N1 viruses have been reported to WHO from primarily 15 countries in Asia, Africa, the Pacific, Europe and the Near East since November 2003. Indonesia, Vietnam and Egypt have reported the highest number of human HPAI Asian H5N1 cases to date.

The first report of a human infection with Asian H5N1 in the Americas was in Canada on January 8, 2014 and occurred in a traveler recently returning from China. Although human infections with this virus are rare, approximately 60% of the cases have died.

Asian HPAI H5N1 Virus Infection Can Cause Severe Illness in People

Asian HPAI H5N1 viruses have infected the respiratory tract of humans, causing severe illness (e.g. pneumonia and respiratory failure) and death in some people.

The majority of human infections with Asian HPAI H5N1 have occurred among children and adults younger than 40 years old. Mortality has been highest in people aged 10-19 years old and in young adults. Most human Asian HPAI H5N1 cases have presented for medical care late in their illness and have been hospitalized with severe respiratory disease. However, some clinically mild cases have been reported, especially in children.

Of the few avian influenza A viruses that have crossed the species barrier to infect humans, Asian HPAI H5N1 virus has caused the largest number of detected cases of severe disease and death in humans. However, it is possible that the most severely ill people are more likely to be diagnosed and reported, while milder cases are less likely to be detected and reported. Despite the high mortality, human cases of Asian HPAI H5N1 remain rare to date, even among persons exposed to infected poultry.

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Most Cases of Asian H5N1 in People Have Been Linked to Contact with Infected Poultry

In the majority of cases, people got HPAI Asian H5N1 virus infection after direct or close contact with sick or dead poultry that were infected with the virus.

Other risk factors include visiting a live poultry market and having prolonged, unprotected close contact with patients infected with HPAI Asian H5N1.. For some HPAI Asian H5N1 cases, the source of exposure is unknown.

The timing of many human cases of HPAI Asian H5N1 has corresponded to the seasonality of HPAI Asian H5N1 virus outbreaks among poultry. Poultry outbreaks occur more often during relatively cooler periods found in months at the beginning and end of the year. However, human cases can occur at any time, especially in countries where HPAI Asian H5N1 is endemic in poultry. Currently, HPAI Asian H5N1 virus is considered endemic in poultry in six countries (Bangladesh, China, Egypt, India, Indonesia, and Vietnam), although other counties have experienced poultry outbreaks.

Currently, HPAI Asian H5N1 virus does not transmit efficiently from person to person. Some cases of limited, non-sustained human-to-human transmission have likely occurred.

Clusters of human Asian HPAI H5N1 cases, (usually 2 cases but ranging from 2-8 cases per cluster), have been identified in several countries. Nearly all of the cluster cases have occurred among blood-related family members, especially those living in the same household. Whether such infections are related to genetic or other factors is currently unknown. While most people in these clusters have been infected with Asian HPAI H5N1 virus through common source exposures such as direct or close contact with sick or dead poultry or wild birds, limited non-sustained human-to-human transmission of Asian HPAI H5N1 virus cannot be excluded and likely occurred in some clusters. In cases where limited human-to-human transmission of Asian HPAI H5N1 virus is thought to have occurred, spread has occurred after a very long period of unprotected close contact (hours in length) with a very sick blood-related family member (e.g., mother-daughter or brother-brother). This has been reported to have occurred in a home and in a hospital room.

For example, a report in 2004 from Thailand, describes probable limited human-to-human HPAI Asian H5N1 virus spread in a family resulting from prolonged and very close contact between an ill child and her mother in a hospital room.1 Limited person-to-person spread of Asian HPAI H5N1 virus from a son to his father in a hospital room was also reported in China in 2007.2 In 2006, WHO reported evidence of non-sustained human-to-human-to-human Asian HPAI H5N1 virus spread in Indonesia. In this situation, eight people in one family were infected. The first family member is thought to have become ill through contact with infected poultry. This person then infected six family members through close prolonged contact. One of those six people (a child) then infected another family member (his father). All of the cases were blood-related family members. No further spread outside of the exposed family was identified.3 Finally, human-to-human transmission of Asian HPAI H5N1 virus was also reported in Pakistan among three brothers in 2007.4

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Additional Resources


  1. Ungchusak K, Auewarakul P, Dowell SF, et al. Probable person-to-person transmission of avian influenza A(H5N1). N Engl J Med. 2005:352(4):333-40.
  2. Wang H, Feng Z, Shu Y, et al. Probable limited person-to-person transmission of highly pathogenic avian influenza A(H5N1) virus in China. Lancet. 2008: 371(9622):1427-34.
  3. World Health Organization. Avian influenza situation in Indonesia update 16. 31 May 2006.
  4. World Health Organization. Weekly Epidemiological Record [1.1 MB, 8 pages]. 2008:40(83):357-364.

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