Subtypes of HIV

There are two main subtypes of HIV, known as HIV type 1 (HIV-1) and HIV type 2 (HIV-2). These subtypes have distinct genetic differences and are associated with different epidemiological patterns and clinical characteristics. HIV-1 exhibits a genetic relation to viruses indigenous to chimpanzees and gorillas that inhabit West Africa, while HIV-2 viruses are affiliated with viruses present in the sooty mangabey, a vulnerable West African primate. HIV-1 viruses can be further stratified into groups M, N, O, and P. Among these, HIV-1 group M viruses are the most prevalent, infecting nearly 90% of people living with HIV and are responsible for the global AIDS pandemic. Group M can be further subdivided into subtypes based on genetic sequence data. Certain subtypes are known for their increased virulence or drug resistance to different medications used to treat HIV. HIV-2 viruses are generally considered to be less virulent and less transmissible than HIV-1 M group viruses, although HIV-2 is also known to still cause AIDS. One of the prevailing challenges in the pursuit of effective management of HIV is the virus's pronounced genetic variability and rapid viral evolution.

Major types

HIV-1

HIV-1 is the most common and most pathogenic strain of the virus. As of 2022, approximately 1.3 million such infections occur annually. Scientists divide HIV-1 into a major group (group M) and two or more minor groups, namely groups N, O and possibly a group P. Each group is believed to represent an independent transmission of simian immunodeficiency virus (SIV) into humans, excluding subtypes within a specific group. The complete genome sequence of HIV-1 contains a total of 39 open reading frames (ORFs) across all six possible reading frames (RFs), but only a few of them are functional.

Group M With 'M' for "major", this is by far the most common type of HIV, with more than 90% of HIV/AIDS cases caused by infection with HIV-1 group M viruses. This major HIV, which was the source of pre-1960 pandemic viruses, originated in the 1920s in Léopoldville, the Belgian Congo, today known as Kinshasa, which is now the capital of the Democratic Republic of Congo (DRC). Its zoonotic origin is the SIVcpz strain, which infects chimpanzees. The M group is subdivided further into clades, called subtypes, that are also given a letter. There are also "circulating recombinant forms" or CRFs derived from genetic recombination between viruses of different subtypes which are in addition each given a number. CRF12_BF, for example, is a recombination between subtypes B and F.

Subtype A is common in parts of eastern Africa, Russia, and former Soviet states. Subtype B is the dominant form in Europe, the Americas, Japan, and Australia. In addition, subtype B is the most common form in the Middle East and North Africa. It may have been exported from Africa when Haitian professionals visited Kinshasa in the 1960s and brought it to Haiti in 1964. Subtype C is the dominant form in Southern Africa, Eastern Africa, India, Nepal, and parts of China. Subtype D is generally only seen in Eastern and Central Africa. Subtype E was originally used to describe a strain that is now accounted for as the combined strain CRF01_AE. This means the original, singular, E strain has disappeared, but we know it existed, as it is visible in this combined strain form. Subtype F has been found in central Africa, South America, and Eastern Europe. Subtype G (and the CRF02_AG) have been found in Africa and central Europe. Subtype H is limited to central Africa. Subtype I was originally used to describe a strain that is now accounted for as CRF04_cpx, with the cpx short for "complex" recombination of several subtypes. Subtype J is primarily found in North, Central, and West Africa, and the Caribbean Subtype K is limited to the DRC and Cameroon. Subtype L is limited to the DRC. The spatial movement of these subtypes moved along the railways and waterways of the DRC from Kinshasa to these other areas. These subtypes are sometimes further split into sub-subtypes such as A1 and A2 or F1 and F2. In 2015, the HIV strain CRF19, a recombinant of subtype A, subtype D, and subtype G, with a subtype D protease, was found to be strongly associated with rapid progression to AIDS in Cuba. This is not thought to be a complete or final list, and further types are likely to be found.

Group N The 'N' stands for "non-M, non-O". This group was discovered by a Franco-Cameroonian team in 1998, when they identified and isolated the HIV-1 variant strain YBF380 from a Cameroonian woman who died of AIDS in 1995. When tested, the YBF380 variant reacted with a viral envelope antigen from SIVcpz rather than with those of Group M or Group O, indicating it was indeed a novel strain of HIV-1. As of 2015, fewer than 20 Group N infections have been recorded.

Group O The O ("Outlier") group has infected about 100,000 individuals located in West-Central Africa and is not usually seen outside of that area. It is reportedly most common in Cameroon, where a 1997 survey found that about 2% of HIV-positive samples were from Group O. Its zoonotic origin is SIVgor, which infects gorillas (rather than the more common source, SIVcpz). The group caused some concern because it could not be detected by early versions of the HIV-1 test kits. More advanced HIV tests have now been developed to detect both Group O and Group N.

Group P In 2009, a newly analyzed HIV sequence was reported to have greater similarity to SIVgor, than SIVcpz. The virus had been isolated from a Cameroonian woman residing in France who was diagnosed with HIV-1 infection in 2004. The scientists reporting this sequence placed it in a proposed Group P "pending the identification of further human cases".

HIV-2 HIV-2 is mostly found in Africa, and therefore less recognized elsewhere in the world.

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