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Efficient Transmission of H5N1 to Humans in Vietnam
March 30, 2005
>> Two other Vietnamese suspects, a 41-year-old woman and a child, had fallen sick in the same district near the northern port city of Haiphong where five members of one family have been confirmed as suffering from bird flu.
"Their conditions are stabilizing," a doctor in Haiphong's Viet Tiep hospital said of the a couple and their three daughters.
The 41-year-old woman was a neighbor of the family and tests on her were under way, he said. Doctors said the five patients ate sick chicken when half of their 400 birds died earlier this month.
"A field investigation of this family cluster is under way," said a World Health Organization statement seen on Wednesday on its Web site .
The U.N. health agency noted recent outbreaks of human cases in Vietnam included several clusters, mostly family members. "There is currently no evidence that the H5N1 virus is spreading easily from person to person," it said, referring to fears the H5N1 virus could mutate into a form which could pass between humans easily and cause a global pandemic. <<
Since the WHO comment on absence of evidence for easy person to person spread is being widely distributed and being used as a headline, a review of the evidence is in order.
H5N1 in a family of five who were simultaneously admitted to a hospital is without precedence and represents a clear change in the efficiency of transmission of H5N1 to persons. In the prior H5N1 person to person transmissions, the evidence was overwhelming, but the transmissions were not sustained. In virtually all clusters, the transmission was limited to family members and usually involved one index case and one or two secondary cases. The secondary cases had a history of contact with the index case, and most of the secondary cases were females, which is consistent with their role of caregiver in the family. These familial clusters were common, account for over one third of reported H5N1 cases. One of these clusters expanded to one or two health care workers. The chain became longer, but again the infection was one person at a time.
The family of five is unique both in size and unimodal onset dates. Details on disease onset dates have not been given, but all five members entered the hospital on the same day, March 22, suggesting a common exposure. Although there has been speculation on a meal, such speculation has little factual support. Most of the evidence in earlier clusters was inconsistent with a common source in general, because the clusters were bimodal, and inconsistent with a specific meal, because in addition to significant differences in onset dates, others who ate the same meal did not get sick or show evidence of H5N1 exposure.
If a meal was the source of the H5N1, it would still be the most efficient transmission, because all five members were infected at about the same time. However, the fact that two or three neighbors also developed symptoms, casts serious doubt on the meal, but does raise issues of a common environmental factor such as airborne transmission of H5N1.
Such transmission for SARS CoV has been recently published, although WHO insisted throughout the SARS pandemic that such transmission was not possible or extremely rare. The evidence was overwhelming to the most casual observer, but numerous press releases and media reports denied the obvious. There is also a report indicating that the H7N3 avian influenza outbreak in British Columbia last season was influenced by airborne virus generated by fans in barns on farms.
The proximity of the two or three neighbors to the family of five is not given other to indicate that all are from the same commune and are neighbors. An airborne transmission would be hard to prove or disprove and a meal would be virtually impossible to prove or disprove.
Clearly, infection of five of five family members is an example of transmission efficiency.
Issuing press releases in the absence of data does not affect transmission of H5N1 to humans, which has clearly increased.