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Proof of Human to Human Transmission of Bird Flu in Vietnam
April 17, 2005
>> Peter Horby, the WHO medical epidemiologist in Hanoi, said that while the mortality rate from H5N1 infections was falling, the virus appeared to be adapting to human hosts, which was taken to be an indication that transmission between humans would become easier.
"The fact that it's been around for a year and we haven't seen a pandemic is no reason to be complacent," he said.
"I'm more concerned than I was a year ago." He estimated the probability of a pandemic at "more than 50 per cent".
Dr Klaus Stohr, the WHO global influenza programme chief, reported an increase in the number of cluster cases reported recently, with the biggest a family of five cases.
There have been seven cluster cases in Vietnam, all within single families, most recently in the northern province of Haiphong.
No relative has been proved to have passed the disease to another, but Dr Horby said: "The onset dates could be consistent with human to human transmission." <<
The above comments by Peter Horby and Klaus Stohr dance around the human-to-human transmission issue. They cite evidence for more efficient human-to-human transmission, but then try to limit the implications by saying that the proven transmissions are not proven.
The most straightforward proof of human-to-human transmission is bimodal distribution of onset dates. When two people who have contact with each other develop symptoms at different times, the simplest explanation is that the patient who developed symptoms at an earlier date transmitted bird flu to the contact that developed symptoms at a later date.
All of the relevant facts are acknowledged. The number of clusters has increased, as have their size. The relevance of the bimodal distribution is acknowledged by the statement that "onset dates could be consistent with human-to-human transmission".
Use of "could" was appropriate last year, when the number of clusters was smaller. However, now the number of familial clusters has grown to more than double the seven acknowledged above and virtually all have bimodal distributions, which as an aggregate provide overwhelming evidence for human-to-human transmission. It is misleading to continue to use "could". The onset dates in virtually all of the familial clusters PROVE human-to-human transmission.
Moreover, the statement that the cluster cases are all within single families is simply false. The cluster in Thai Binh involving the two nurses extended beyond a single family. The brother, sister and their nurse were all confirmed to be H5N1 positive, and the negative tests on the second nurse remain suspect.
The number of confirmed human cases remains very low, and symptoms in a second nurse in the same hospital as the three confirmed cases raises the suspicion level regarding infection to a high level. Although the nurse tested negative twice, there are no reports on an etiological agent that caused the bird flu symptoms in the second nurse.
When top WHO representatibves define evidence for human-to-human transmission, but fail to acknowledge the existence of human-to-human transmissions proven by the clear evidence, the number of reasons why the monitoring and control of avian influenza in Asia is scandalous, increase.