Almost all HIV-positive people in the UK are infected with HIV-1. HIV-1 originates from the transfer of the simian immunodeficiency virus from the chimpanzee subspecies Pan troglodytes troglodytes to humans.1 At least three separate zoonotic transmissions resulted in the formation of three distinct HIV-1 groups: M (main), O (outlier), and N (non-M/non-O).
About 90% of HIV-1 infections are classified as group M and these are distributed worldwide. Group O infections are endemic to several west central African countries and represent 1 to 5% of all HIV-1 infection in those areas. Group N has only been identified in a small number of individuals in Cameroon.
Within the HIV-M group, there is a further division into at least ten subtypes or clades (groups of genetically related virus). Historically, the distribution of subtypes followed the geographic patterns listed below.
- Subtype A: Central and East Africa as well as East European countries that were formerly part of the Soviet Union.
- Subtype B: West and Central Europe, the Americas, Australia, South America, and several southeast Asian countries (Thailand, and Japan), as well as northern Africa and the Middle East.
- Subtype C: Sub-Saharan Africa, India, and Brazil.
- Subtype D: North Africa and the Middle East.
- Subtype F: South and southeast Asia.
- Subtype G: West and Central Africa.
- Subtypes H, J, and K: Africa and the Middle East.
Additionally, different subtypes can combine genetic material to form a hybrid virus, known as a 'circulating recombinant form' (CRFs), of which at least twenty have been identified.2
Earlier in the HIV pandemic, dominant clade types and common routes of transmission were identified in specific geographic areas, e.g. in southern Africa, subtype B infection was linked to homosexual transmission and subtype C to heterosexual transmission.
Assumptions can no longer be made concerning transmission patterns for particular clades. While heterosexual transmission drives most subtype A HIV-1 infections in sub-Saharan Africa, injecting drug use is strongly linked to subtype A infection in Eastern Europe.
Subtype B is the most common form of HIV-1 found in the UK across all routes of transmission. However, through subtyping nearly 900 specimens collected in an ongoing national unlinked anonymous HIV surveillance programme in 1997 and extrapolating those results to 1998 prevalence reports, researchers concluded that 27% of HIV diagnoses in the UK were non-subtype B and occurred mainly through heterosexual contact.3 Onward transmission has resulted in some non-subtype B infections being identified amongst gay and bisexual men.4 5 6
Certain viral subtypes are transmitted more frequently than others in various geographic areas, but a consistent linkage pattern between subtype and transmission route overall cannot be made, as the following studies illustrate.
One study of serodiscordant couples in Uganda found that subtype A was transmitted nearly twice as often as subtype D, but reasons for this could not be identified.7
In India, a study comparing the relative transmission efficiency of HIV-1 subtypes A and C in vitro, showed that subtype C isolates were able to replicate and transmit (higher transmission efficiency across cervical mucosa) than subtype A isolates.8
In southern Brazil, a study of mother-to-child transmission amongst women with either subtype C (the predominant subtype in that area) or subtype B found that subtype did not increase the risk of vertical transmission.9
The reliability of HIV tests for different subtypes is covered in a later section.