More on H2H H5N1 Transmission Media Myth
Recombinomics Commentary 14:26
April 9, 2008
The recent WHO update on the extended human to human to human (H2H2H) transmission in Pakistan, as well as the Lancet paper on H2H in China has focused media attention on H2H transmission, which has been the subject of a media myth for the past four years. This was due in part by efforts of various governments and WHO to minimize the number of examples, which is more than several dozen. A great deal of effort focuses on splitting hairs between a familial cluster due to a common source (poultry) and clusters due to H2H transmission. This distinction is not significant, since both signal a more efficient transmission, which is the key requirement for a catastrophic pandemic that could greatly exceed 1918.
Currently H5N1 can grow well and produce fatal infections in a wide range of mammalian host, including humans. In humans, the case fatality rate for WHO confirmed cases exceeds 60%, which if coupled to the transmission efficiency of seasonal flu, would generate 100’s of millions of fatalities.
The media, generally does not consider a gradual change in transmission efficiency, so the mention of H5N1 and H2H in the same sentence creates concerns. However, limited H2H has been clear since the current expansion, which began in late 2003, was reported. Most of the confirmed clusters have been limited by the lack of sample collection from the index case. Those clusters which involve family members with links to poultry have been excluded from the list of proven examples, so H2H examples are usually limited to a handful of examples, when in fact the number of clusters is closer to 50 and the vast majority of such clusters involve H2H.
In fact, most initial confirmed human cases in country belong to a cluster. These cluster countries include Cambodia, Indonesia, China, Turkey, Iraq, Azerbaijan, and Nigeria. In Turkey, the number of lab confirmed cases was 21 and almost all were in clusters, although only 12 were subsequently confirmed by WHO labs because of sample degradation. Similarly, about 50% of the 2005 cases in Indonesia were in clusters, as were approximately 1/3 of the cases in Vietnam in 2005. Moreover, the discounting of clusters, due to lack of sample collection of misdiagnosis, is still ongoing, as seen in recent clusters in Indonesia.
The media myth, concerning the frequency of such clusters, continues, leading many to assume that the recent clusters in Pakistan and China are usual. Moreover, lack of human cases in India and Bangladesh remain suspect, and the clusters in Indonesia remain unconfirmed, due in part to the absence of sample testing, as well as false negatives due to collection of samples after the start of Tamiflu treatment.
Thus, the frequent and size of clusters is grossly under-estimated, and readers of the popular press remain surprised by the recent discussions of H2H transmission. Moreover, H2H transmission in families is also couched in terms of a genetic predisposition, for which there is no real data. Currently, H5N1 transmission is inefficient and requires close contact, which is most common among family members. In larger clusters there are examples of contacts who are not blood relatives, including husband / wife, friends, and patient / nurse. Moreover H5N1 is largely a avian virus at this time, but has been isolated from a wide range of mammals including domestics and wild cats, dogs, stone martens, foxes, Civet cats, swine, and mice, so the likelihood of a significant human predisposition remains remote.
However, the continued passage of H5N1 through a wide variety of mammals, including humans, increases the likelihood of the acquisition of polymorphisms via recombination, leading to more efficient transmission to humans.
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