Genetic Strains of HIV-1 and HIV-2

One of the primary barriers to treating or developing effective vaccine for HIV is the high genetic diversity of the virus itself. While viruses that use double-strand DNA to replicate are relatively stable, retroviruses like HIV go backward in their replication cycle (using single-strain RNA) and are far less stable. As a result, HIV is highly prone to mutation—mutating, in fact, about a million times more frequently than cells using DNA.

As the virus' genetic diversity widens and different viral subtypes are passed from person to person, the mixed genetic material can create new HIV hybrids. While most of these hybrids die, the few surviving ones often exhibit greater resistance to HIV therapy and, in some cases, faster disease progression.

The variability of HIV, therefore, creates something of a "moving target" for researchers, with new recombinant (combined genetic) strains able to resist or altogether evade neutralizing agents. Some, like the A3/02 strain identified by Swedish researchers in 2013, are able to deplete a person's immune defenses far more aggressively than previously known strains.

What Are HIV-1 and HIV-2?

There are two types of HIV: HIV-1 and HIV-2. HIV-1 is considered the predominant type, representing the vast majority of infections worldwide, while HIV-2 is far less common and primarily concentrated in the west and central African regions. While both of these HIV types can lead to AIDS, HIV-2 is much more difficult to transmit and far less virulent than HIV-1.

Within each of these HIV types are a number of groups, subtypes ("clades"), and sub-subtypes. Undoubtedly, other subtypes and recombinant strains will be discovered as the global spread of HIV continues.

HIV-1 Groups and Subtypes

HIV-1 is divided into four groups: Group M (meaning "major"); Group O (meaning "outlier," or beyond where other groups are seen); and Group N (meaning "non-M" and "non-O"); and Group P (meaning "pending"). The four different groups are classified by the four different simian immunodeficiency viruses (SIV) that were known to be passed from apes or chimpanzees to man.

HIV-1 Group M

HIV-1 Group M was the first group to be identified and today represents about 90% of HIV cases worldwide and can be found virtually in every part of the planet. Within this group are 10 subtypes, which can be stratified by, among other things, their geographic distribution and their impact on different risk groups.

• Subtype A: seen in West Africa and affecting primarily heterosexuals and injecting drug users (IDUs).
• Subtype B: the predominant subtype in Europe, the Americas, Japan, Thailand and Australia, representing nearly all infection in North America and about 80% of all in Europe. Infections occur more among men who have sex with men (MSM) and IDUs than heterosexuals.
• Subtype C: described as the most prevalent HIV subtype representing 48% of all infections worldwide, primarily heterosexual and primarily in Sub-Saharan Africa, India and parts of China.  
• Subtype D: isolate mainly in East and Central Africa.
• Subtype E: a subtype only seen in a recombinant form with subtype A.
• Subtype F: among a smaller percentage of infections seen in Central Africa, South America and Europe.
• Subtype G: among a smaller percentage of infections seen in parts of Africa and Europe.
• Subtype H: among a smaller percentage of infections seen in Central Africa.
• Subtype J: observed in North, Central and West Africa, and the Caribbean
• Subtype K: limited to the Democratic Republic of Congo (DRC) and Cameroon.

HIV-1 Group O

HIV-1 Group O was discovered in 1990 and represents only 1% of the infections worldwide. This HIV group is isolated in Cameroon and neighboring African countries.

HIV-1 Group N

HIV-1 Group N was discovered in 1998 and, again, has only been seen in Cameroon with less than 20 cases documented to date.

HIV-1 Group P

HIV-1 Group P is a rare type of HIV, first identified in a woman from Cameroon in 2009. It can be differentiated from other HIV groups insofar as its origins have been linked to a form of SIV found in western gorillas. Although the "P" classification was meant to infer a "pending" status (i.e., awaiting confirmation of additional infection), a second documented case was identified in 2011 in a Cameroonian man.

HIV-2 Groups

Although cases of HIV-2 have been identified elsewhere, infections are almost exclusively seen in Africa. There are currently eight HIV-2 groups, although only subtypes A and B are the only ones considered epidemic. HIV-2 is believed to have crossed species from a type of SIV affecting the sooty mangabeys monkey directly to humans.

HIV-2 Group A is mainly seen in West Africa, although international travel has led to a small handful of documented cases in the U.S., Europe, Brazil, and India. By contrast, HIV-2 Group B has been confined to parts of West Africa.