- "Fuelling the suspicion hanging
over person-to-person transmission is the unusually long time lag of 15
days between the first and the last person in the cluster falling ill.
- The incubation period for the H5N1 is
usually no more than seven days and if the family had been exposed to the
same source, they would all have fallen ill at about the same time.
- "You want to look at the dates of
onset of the disease. If they are close together they may have had the
same exposure," Thompson said.
- The above comments by WHO on the importance
of disease onset dates in determining to source of H5N1 bird flu are also
used to identify human-to-human transmission clusters. The onset
dates are the most important data point because most H5N1 infections have
an incubation time of 2-5 days as indicated in the New England Journal
of Medicine review of H5N1 cases, authored by physicians who are WHO consultants.
The 2-5 dates contradict recent comments to the New York Times that the
incubation time for H5N1 was normally 7-10 days. The shorter time
explains why most of the H5N1 clusters reported since 2004 have a 5-10
day gap between the index case and other family members. Half of
the gap is due to the incubation time in the newly infected patients, and
half is due to the time it takes for the index case to be optimally contagious.
- Because of the importance of disease
onset dates, they are usually included in WHO updates. They were
withheld from descriptions of the Turkey clusters involving the two largest
families, who were cousins and also withheld in the Sumatra cluster.
Although onset dates were given for the first and last victim, there were
no dates for the other H5N1 confirmed cases.
- Recently, the number of suspected human-to-human
clusters acknowledged by WHO has been rising. However, the vast majority
of prior clusters have the 5-10 gap in disease onset dates, indicating
most are due to human-to-human transmission, since bird to human infections
are very rare and two independent infections from birds are unlikely.
Authors from the WHO and the CDC wrote a report on the first 15 clusters,
and most had the 5-10 day gap. The report was on cluster through
there first cluster in Indonesia. Subsequent clusters have shown
the same pattern, and clusters are common in Indonesia.
- However, WHO investigates these cases,
focusing on non-human sources and if found assuming the non-human source
infected all family members, instead of just the index case. Thus,
the presence of infected birds in the area does not explain the time gap,
which is most easily explained by human-to-human transmission.
- © 2006 Recombinomics. All