Human-to-human transmission of avian influenza

At present, is there evidence of efficient human-to-human transmission?

There have been a limited number of well documented cases in which there is evidence to suggest human-to-human transmission but to date there is no evidence that the highly pathogenic avian influenza virus has adapted to spread easily in humans.

Examples of these family clusters are described below:

1. In Thailand, on 27 September 2004 the Ministry of Health announced possible human-to-human transmission in a family cluster. Thai officials have concluded that the mother could have acquired the infection either from some environmental source or while caring for her daughter, and that this represents a probable case of human-to-human transmission. While the investigation of this family cluster provides evidence that human-to-human transmission may have occurred, evidence to date indicates that transmission of the virus among humans has been limited to family members and that no wider transmission in the community has occurred. Continued vigilance is needed to determine whether the epidemiological situation in humans remains stable.

2. Laboratory confirmed avian influenza HPAI H5N1infection was documented in two brothers in northern Viet Nam. The first case, a 46-year-old resident of Thai Binh Province, developed symptoms on 01 January 2005. He died on 09 January 2005.

His 42-year-old brother, a resident of Hanoi, developed symptoms on 10 January 2005, nine days after his brother fell ill. He was hospitalized in Hanoi and recovered from his illness. He is known to have provided bedside care for his brother, who was treated at the same hospital in Hanoi. A third brother, aged 36 years, who was hospitalized for observation only, did not develop symptoms. The investigation surrounding the new cases considered two hypotheses.

The first included the possibility that the 42-year-old man may have acquired his infection directly from his brother. All evidence to date suggests that isolated instances of limited, unsustained human-to-human transmission can be expected from avian influenza viruses in humans. The second hypothesis focused on a possible direct source of poultry-to-human transmission. Preliminary findings pointed to a family meal in which a dish containing raw duck blood and raw organs was served.

3. Between 27 April and 22 May 2006, a family cluster of HPAI H5N1 occurred in the village of Kubu Sembelang, Karo District, of North Sumatra. Indonesia. The outbreak involved seven members of an extended family, all but one of whom died. All cases in the cluster can be directly linked to close and prolonged exposure to a patient during a phase of severe illness. Although human-to-human transmission cannot be ruled out, the search for a possible alternative source of exposure is continuing.

Epidemiological investigation of other contacts within the family and also health care workers did not show any new cases of influenza. Based on an assessment of present evidence, WHO has concluded that the current level of pandemic alert is appropriate and does not need to change. The level of pandemic alert remains at phase 3. This phase pertains to a situation in which occasional human infections with a novel influenza virus are occurring, but there is no evidence that the virus is spreading in an efficient and sustained manner from one person to another.

Does human infection with HPAI H5N1 happen often?

No. Only very rarely. The first documented human infections with the HPAI H5N1 avian strain occurred in Hong Kong in 1997. In that first outbreak, 18 persons were hospitalized and 6 of them died. The source of infection in all cases was traced to contact with diseased birds on farms (1 case) and in live poultry markets (17 cases). The human cases coincided with outbreaks of HPAI H5N1 in poultry.

In February 2003, the HPAI H5N1 strain again jumped from birds to infect two members of a family (a father and his son) when they returned to Hong Kong following travel in southern China. The father died but the son recovered. A third member of the family, the boy's sister, died of a severe respiratory illness in China. No samples were available for determining the cause of her death.

There have since been further cases of human infection with HPAI H5N1 across South East Asia, and also in the Middle East, Africa and Eastern Europe. More information is available on the Avian Flu News page of the HPA website.

Is it reassuring that so few human cases have occurred?

Yes. WHO has some evidence that the HPAI H5N1 strain may have been circulating in birds since April 2003. The detection so far of only a few human cases suggests that the virus may not be easily transmitted from birds to humans at present. However, the situation could change quickly, as the HPAI H5N1 strain has been shown to mutate rapidly and has a documented propensity to exchange genes with influenza viruses from other species.

In situations that could favor the emergence of a new pandemic strain of influenza virus, every case of human infection is one too many. In addition to the rapid destruction of infected animals, another opportunity to prevent human cases is through the protection of workers involved in culling operations. WHO has issued guidelines for conducting these operations safely. 

Last reviewed: 13 August 2008