Some important epidemiological features of human H5N1 infections occurred in northern Viet Nam during January through April 2005 and appeared to differ in some respects from those seen in 2004 in other parts of Asia, and in the concurrent period in southern Viet Nam. These included an increase in the number of case clusters in the north compared with the south, a prolonged interval between the first and last cases in clusters, detection of sub-clinical infections, an expanded age range of cases and fewer fatal cases. Investigators were not able to prove that human-to-human transmission had occurred.

However they expressed concerns, which were shared by local clinicians, that the pattern of disease appeared to have changed in a manner consistent with this possibility.

These differences suggest that the epidemiology of H5N1 infections may be evolving in Asia. The changes in epidemiological patterns are consistent with the possibility that recently emerging H5N1 viruses may be more infectious for humans.

Furthermore, sequencing analyses of H5N1 genes from avian and human H5N1 viruses from several countries suggest that they are becoming more antigenically diverse and may be forming distinguishable groupings based on phylogenetic analyses.

While the implications of these epidemiological and virological findings are not fully clear, they demonstrate that the viruses are continuing to evolve and pose a continuing and potentially growing pandemic threat. Based on these concerns and findings, it would be prudent to take  increased steps to improve risk assessment procedures, to strengthen the ability of affected countries to respond promptly to local outbreaks, to accelerate control of avian influenza in poultry and to implement or complete pandemic preparatory actions as soon as is possible, even if current H5N1 outbreaks in Asia cease or diminish during the summer.

2.2 Supporting viral observations
?In general, H5N1 viruses isolated during 2004 from humans with severe respiratory infections
were very similar to avian isolates from the same country, both genetically and antigenically.
H5N1 human and avian viruses isolated in the Indochina peninsula (Cambodia, Laos, Malaysia,
Thailand and Viet Nam) tightly clustered within clade 1, while H5N1 viruses isolated from
birds in China, Indonesia, Japan and South Korea belonged in a second clade which showed
greater genetic divergence.
?The HA gene of viruses isolated from humans in Viet Nam in the first 3 months of 2005
showed several amino acid changes relative to 2004 viruses. None of the changes in the HA
were common to all of the 2005 viruses analysed so far. However, the most commonly
observed changes are located close to the receptor binding site and could potentially modulate
receptor binding specificity. Recent viruses circulating in Northern Viet Nam have lost an
arginine residue in the mutibasic amino cluster at the proteolytic cleavage site of the HA
protein. It does not seem to be responsible for reduced pathogenicity since the structure of the
cleavage site still remains typical of highly pathogenic viruses. However at this time these are
possibilities that have yet to be further investigated by analysing more viruses.
?Phylogenetic analysis of all H5N1 human isolates along with a subset of avian viruses from
clade 1 isolated during 2004 and 2005 in Cambodia, Thailand and Viet Nam indicates that the
viruses from northern Viet Nam and Thailand have begun to form a somewhat separate cluster
from viruses isolated from southern Viet Nam and Cambodia. As of yet, the 慴ootstrap?values
(which provides a measure of certainty with which clades can be accurately separated) are low
due a lack of data. Sequences from additional viruses from the northern and southern regions of
Viet Nam would add statistical power to the phylogenetic analysis.
?H5N1 human isolates from Viet Nam in 2005 are somewhat antigenically heterogeneous . In
particular, A/Viet Nam/JPHN3021/2005 is antigenically distinct from the 2004 reference/
vaccine strains A/VN/1203/04 and A/VN/1904/04. Other 2005 H5N1 human isolates exhibit
more antigenic heterogeneity than did those isolated in 2004.
?Sequence analysis of NA genes and neuraminidase inhibitor susceptibility testing of H5N1
human 2005 isolates from Viet Nam has also revealed that one virus (A/VN/HN30408/05) has a
搈ixed?virus population of amino acid residues 274-H (wild-type) and 274-Y (resistant)
sequences. Although this virus does not exhibit a fully resistant phenotype, the IC50 value for
oseltamivir is shifted upward and consequently this virus is less susceptible to oseltamivir than
other H5N1 isolates tested. The patient from whom this virus was isolated had been/was being
treated with oseltamivir. The community emergence and spread of viruses resistant to
oseltamivir, if it were to occur, would have significant implications for influenza A/H5N1
prevention and control.

In summary, analysis of the virologic data has identified trends in phenotypic and genotypic
properties of 2005 human isolates that must be examined more closely using additional recent
human and avian H5N1 isolates. It is essential that avian and human isolates are included and
compared to determine if the 2005 viruses isolated from humans simply reflect the genetic and
antigenic diversity found among H5N1 viruses obtained from poultry or if viruses isolated from
humans are beginning to exhibit antigenic drift from avian viruses. The latter pattern would be of
major concern since it would likely indicate that a higher level of human-to-human transmission is
occurring. Finally, while the antigenic and genetic data indicate that the 2005 human isolates are
drifting antigenically and genetically from their 2004 predecessors and the H5N1 viruses isolated
from the northern and southern parts of Viet Nam tend to fall into different groups, there is no
direct evidence to date that these changes can be correlated with apparent changes in disease
patterns between the north and the south. Additional data from 2005 human and avian H5N1
isolates must be obtained and analyzed together with data from older H5N1 viruses.