What is the difference betweenHIV-1 and HIV-2?
Two types of HIV are currently recognized: HIV-1 and HIV-2. Worldwide, the predominant virus is HIV-1. Both types of virus are transmitted by sexual contact, through blood, and from mother to child, and they appear to cause clinically indistinguishable AIDS. However, HIV-2 is less easily transmitted, and the period between intitial infection and illness is longer in the case of HIV-2.
How many subtypes do we know about?
We currently know of at least 10 genetically distinct subtypes of HIV-1 within the major group (group M) containing subtypes A to J. In addition, group O (Outliers) contains a distinct group of very heterogeneous viruses. These subtypes are unevenly distributed throughout the world. For instance, subtype B is mostly found in the Americas, Japan, Australia, the Caribbean and Europe; subtypes A and D predominate in sub-Saharan Africa; subtype C in South Africa and India; and subtype E in Central African Republic, Thailand and other countries of southeast Asia. Subtypes F (Brazil and Romania), G and H (Russia and Central Africa), I (Cyprus), and group O (Cameroon) are of very low prevalence. In Africa, most subtypes are found, although subtype B is less prevalent.
What are the major differences between these subtypes?
The major difference is their genetic composition; biological differences observed in vitro and/or in vivo may reflect this. It has also been suggested that certain subtypes may be predominantly associated with specific modes of transmission: for example, subtype B with homosexual contact and intravenous drug use (essentially via blood) and subtypes E and C, with heterosexual transmission (via a mucosal route). Laboratory studies undertaken by Dr Max Essex of the Harvard School of Public Health in Boston have demonstrated that subtypes C and E infect and replicate more efficiently than subtype B in Langerhans cells which are present in the vaginal mucosa, cervix and the foreskin of the penis but not on the wall of the rectum. These data suggest that HIV subtypes E and C may have a higher potential for heterosexual transmission than subtype B. However, caution should be exercised in applying in vitro-studies to real-life situations. Other variables which affect the risk of transmission, such as the stage of HIV disease, the frequency of exposure, condom use, and the presence of other sexually transmitted diseases (STDs), must also be taken into consideration before any definite conclusions can be drawn.
Are some subtypes more infectious than others?
Some recent studies have suggested that subtype E spreads more easily than subtype B. In one study conducted in Thailand (Mastro et al., The Lancet, 22 January 1994), it was found that the transmission rate of subtype E among female commercial sex workers and their clients was higher than that for subtype B found among a general population in North America. In a second study conducted in Thailand (Kunanusont, The Lancet, 29 April 1995), among 185 couples with one partner infected with HIV subtypes E or B, it was found that the probability of both partners in a couple becoming infected was higher for subtype E (69%) than for subtype B (48%). This suggests that subtype E may be more easily transmissible. However, it is important to note that neither study was designed to fully control for multiple variables which may affect the risk of transmission.
How can one protect oneself against the different subtypes?
The condom and the adoption of safe sex behaviour are still the methods that work best to avoid HIV infection, regardless of subtype.
Is subtype E a new subtype?
Subtype E is not new. Stored blood samples show that subtype E was already identified at the beginning of the epidemic in Central Africa and as early as 1989 in Thailand.
Is subtype E responsible for the rapid spread of HIV in Thailand and is there reason to expect an explosive spread of subtype E in other countries?
Recent findings on the rapid spread of subtype E in Thailand require further confirmation; and other variables that may affect the risk of transmission need to be studied. The possibility of subtype E virus spreading into other countries cannot be excluded. The prevention strategies advocated by UNAIDS which are currently being applied in countries such as Thailand, are valid in all parts of the world. In the event of subtype E spreading in Europe and other industrialized countries, these prevention strategies do not need to be altered, but simply continued and reinforced. As long as people practise safe sex, there is no need for alarm or panic. While UNAIDS cautions that more research needs to be done before the relative infectivity of subtype E can be established, the programme welcomes the current debate. This debate may serve to remind people that it is imperative that preventive behaviour continue to be promoted as long as the epidemic is not conquered in every part of the world.
Do conventional AIDS tests detect all subtypes?
Routine AIDS tests which are currently being used for blood screening and diagnostic purposes detect virtually all subtypes of the human immunodeficiency virus. (Most companies have modified their assays so that they detect the newly identified HIV-1 group O strains.)
Are more subtypes likely to "appear"?
10 subtypes have been identified in the past four years since the techniques to detect subtypes in HIV-1 were introduced in 1992. It is almost certain that new HIV genetic subtypes will be discovered in the future, and that the known subtypes will continue to spread to new areas as the global epidemic continues. For example, two recent articles (Artenstein and Brodine, The Lancet, 4 November 1995) report some cases of persons infected with subtype E in Uruguay and in the United States (apparently from Cambodia and Thailand respectively).
What are the implications of HIV variability for research on treatment and vaccines?
More research needs to be undertaken. Some HIV subtypes have been observed in the laboratory to have different growth and immunological characteristics; these differences need to be demonstrated in vivo. It is not known whether the genetic variations in subtype E or other subtypes actually make a difference in terms of the risk of transmission, the response to antiviral therapy, or prevention by vaccine. If these genetic variations do make a difference in terms of vaccine effectiveness, this indeed could represent a major obstacle to the development of a widely effective or "global" HIV vaccine. The influenza vaccine has to be periodically modified and updated because of the genetic variations of the influenza virus. The same might need to be done with an HIV vaccine. UNAIDS is supporting a global network for HIV isolation and characterization to monitor the distribution and emergence of new subtypes. The information collected is being used to monitor the dynamics of subtype distributions globally and for vaccine research and evaluation.