Thanks for your question on HIV clades. I've done a significant
amount of HIV research over the years, and can answer your question.
As you may know, HIV has the highest mutation rate of any organism
known. This makes the categorization of HIV strains difficult. Also
of interest, different portions of the HIV genome are more variable
than others. The classification of HIV strains depends on which
portion of the HIV genome one is looking at. Generally, the clades
are based on phylogenetic (evolutionary) data on these diverse HIV
strains. The classification and distribution of HIV strains has
obvious implications for vaccine research.
The following reference describes the distribution of clades around the world:
Here is an excerpt from the above site, giving the distribution of
clades by country, as well as the meaning of some of the clade
definitions (those not defined are simply sequential alphabetical
"Viruses of group M (for "main") are responsible for the majority of
infections worldwide; group O (for "outgroup") is a relatively rare
group currently found in Cameroon, Gabon, and France. Group M can be
divided into at least eight distinct subtypes or clades (A through H).
Thus, variation in HIV-1 sequences is not continuous over a spectrum,
but instead appears to cluster into discrete groupings. Isolates of
HIV-1 from different clades may differ by 3040% in the amino acid
sequence of the gp120 SU protein; isolates within a clade vary from 5%
to 20%. It is not clear whether this pattern has resulted from
multiple introductions of virus into humans or from discrete pathways
of evolution or adaptation after the virus entered the human
population. Two chimpanzee isolates occupy a position in the
phylogenetic tree intermediate between the M and O group HIVs,
suggesting that the two principal groupings of HIV-1 represent
separate introductions. The different clades of HIV-1 are not
distributed evenly throughout the world. For example, clade B
predominates in North America and Europe and clade E predominates in
Figure 2 of this document provides a worldwide map of the distribution
of clades, which you may also find useful.
You can also find information on HIV variation at this NIH site:
This site has links to databases of HIV strains as well.
Here is another NIH site with information on HIV variation:
This site includes a map of HIV prevalence by clade (sub-type) across
the globe. The map has pie charts by region, which show the
prevalence of each clade in that particular region.
I hope this information was useful. Feel free to request any clarification.