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More Evidence for Human to Human Transmission of Bird Flu
January 26, 2005
>> As announced previously, a family cluster in northern Viet Nam has been the focus of intense investigation. The cluster involves three brothers. Of these, Vietnamese authorities have identified influenza A H5 infection in two: a 46-year-old man and his 42-year old brother. The older brother developed symptoms on 26 December and died on 9 January. The younger brother was hospitalized with respiratory symptoms on 12 January and has now fully recovered. He is known to have provided bedside care for his older brother during a period of critical illness.
The source of infection for the two brothers remains undetermined and investigations are ongoing. The third brother, aged 36 years, was hospitalized for observation only, did not develop symptoms, and remains in good health. Results of tests, conducted as part of the investigation, are pending. Clinical specimens for the two confirmed cases are being sent to a WHO collaborating centre for further characterization.<<
The information in the latest WHO update would strongly suggest that the 42M was infected by his older brother based on the time differential (17 days) between onset of symptoms. As noted in the recent New England Journal of Medicine (NEJM) article, symptoms usually develop within 2-10 days of exposure. The time differential of greater than 10 days was the primary reason for concluding that human to human transmission had occurred in Thailand between the daughter (11F) and mother (26F). The aunt (32F) probably was infected by her niece or sister (and the niece was most likely because there was no contact between the aunt and mother when the mother had more severe symptoms).
In the NEJM paper, the authors noted that they were fortunate that there was no initial contact between mother and daughter, because most clusters involve both common sources, as well as close contact between relatives.
For the current case in Vietnam, media reports and an earlier WHO report indicate that there was a meal involving blood from an uncooked duck (media reported the meal was on December 29, which would not be a source of symptoms on December 26). A meal prior to December 26 would be an unlikely source for the middle brother because of the long time differential between the meal and symptoms on January 10 or hospitalization on January 12.
The initial media reports on the youngest (36M) brother indicated he was hospitalized on January 20. However, subsequent reports indicated he did not develop symptoms, so it is likely that the H5N1 positive test was on antibody, which would make it difficult to identify the source of the infection for the youngest brother.
The disease onset date of December 26 for the deceased brother, strongly suggests that the 42M was infected while caring for his older brother.