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WHO on Transmission of H5N1 to a Family of Five in Haiphong

Recombinomics Commentary
March 29, 2005

>> WHO is aware of reports of suspected H5 avian influenza infection in five members of a family who are presently hospitalized in the northern port city of Haiphong. These cases, which include the parents and their three young daughters, are undergoing further investigation following initial tests indicating infection with the H5 subtype of avian influenza. Reports indicate outbreaks of avian influenza in poultry in the vicinity. Field investigation of this family cluster is under way.

The current outbreak of human cases in Viet Nam has included several clusters, mostly in family members, of cases closely related in time and place. Thorough investigation of all such clusters is essential to determine possible changes in the behaviour of the virus and thus support assessment of the risk of an influenza pandemic.

There is currently no evidence that the H5N1 virus is spreading easily from person to person. Rapid sharing with WHO of viruses from recent clusters of cases has become increasingly important. Analysis can determine whether any significant changes in the virus have taken place and provide further support for risk assessment. <<

Although WHO appreciates the importance of monitoring cluster changes and obtaining sequences of associated isolates, it is not clear how much information is forthcoming out of Vietnam.

The H5N1 positive family of five and the three hospitalized neighbors are a very major red flag, signaling a change in the transmissibility of H5N1 to humans.  The familial cluster is not only the largest H5N1 cluster to date, it is also easily the largest common source cluster.  The earlier familial clusters were bimodal, indicting limited human-to-human transmission within the family, and more recently, limited transmission to health care worker(s).  However, these transmissions were relatively inefficient and involved 1-2 transmissions at a given time.

Transmission of H5N1 to five (and possibly 8 if the neighbors are linked to the common source) clearly is a new event.  These more recent clusters also appear to be associated with milder cases. Although all family members had fevers and breathing difficulties, media reports indicate all have improved markedly. 

A lower case fatality rate would be positive for individual cases.  However, increased transmissibility to humans from humans or birds could lead to more genetic instability among H5N1 isolates. 

An easily transmitted H5 influenza could signal the rapid evolution of novel recombinants which could produce a pandemic that could have a significant impact on an immunologically naive human population.

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