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Human Bird Flu Transmission in Thai Binh Vietnam

Recombinomics Commentary
March 7, 2005

>> A 21-year-old man from Thai Binh Province. He developed symptoms on 14 February and was admitted to hospital on 20 February.
His 14-year-old sister, also from Thai Binh Province. She developed symptoms on 21 February and was hospitalized the following day. <<

The latest update from WHO clearly shows that the most recent Thai Binh familial cluster was bimodal, indicating more human-to-human transmission of bird flu.  Since the latest transmission of H5N1 from patient to nurse was also in Thai Binh, as were the recent cases, a review of the evidence for human-to-human transmission is useful.

There have now been 11 familial clusters of H5N1 avian influenza in Vietnam, Thailand, and Cambodia.  Since reported bird flu cases are very rare, two cases within the same household strongly suggests a common source, or human-to-human transmission.  Since the family members are infected with identical or closely related H5N1, distinguishing between a common source and human transmission is heavily dependent on onset dates.  Most cases show symptoms 2-4 days after exposure.  Therefore, cases involving a common source will develop symptoms at about the same time.  If the transmission is from one family member to another, there will be a longer time lag and the onset dates will be bimodal.

As noted above, the latest familial cluster from Thai Binh has a 7 day differential between onset dates suggesting the sister was infected by her brother.  A common source, such as a meal of blood pudding is unlikely because of the bimodal onset dates.  However, the blood pudding is an unlikely common source for additional reasons.  The index case developed symptoms 5 days after the meal and his sister developed symptoms 12 days after the meal. So both fall outside of the common 2-4 days of symptoms post exposure.  Moreover, the third sibling who ate the meal did not develop symptoms.  Thus, the blood pudding is an unlikely source because none of the siblings developed symptoms within 2-4 days, none developed symptoms at the same time, and one sibling did not get sick.

The earlier Thai Binh cluster also involved blood pudding and it was an unlikely source for the same reasons. The index case developed symptoms 1 day after the meal.  His brother developed symptoms 14 days after the meal, and the brother-in-law did not develop any symptoms.  The case was also bimodal for onset dates.

Last year there was a third familial cluster in Thai Binh, and again the onset dates were bimodal.  However, in that cluster two sisters were infected by their brother.  The sisters developed symptoms on the same day, entered the hospital on the same day, initially were inconclusive on the H5N1 test, and died on the same day.  Their disease progression was similar because they had a common source, their brother.

Thus, there have been three familial clusters in Thai Binh, and all three were bimodal.  Moreover, the other 8 clusters, including the cluster in Thailand described in the New England Journal of Medicine, were bimodal.  These 11 bimodal clusters account for over one third of the reported H5N1 cases, demonstrating that human-to-human transmission is common.

The onset and hospitalization dates of the latest cluster also point to human-to-human transmission between the two patients and the health care worker who was H5N1 positive.  The brother was admitted to the hospital in Thai Binh on Feb 20 and transferred to Hanoi on Feb 22.  His sister was admitted on Feb 22 and transferred to Hanoi on or about Feb 24.  The health care worker was admitted Feb 28 to the hospital in Hanoi with advanced symptoms, suggesting initial symptoms were a couple of days earlier.  Symptoms likely appeared 2-4 days after exposure to the two H5N1 patients, strongly suggesting they were the source of his infection.

In addition to the familial and hospital clustering, all of the recent reported cases are from northern Vietnam and most are from Thai Binh.  This geographical clustering suggests additional human-to-human transmission in Thai Binh.

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