Facts and Issues [International Conference on AIDS (11th: 1996: Vancouver, Canada)]

Variability of the Virus
Text on this page supplied by the United Nations Program on AIDS (UNAIDS). Re-printed with permission.
What is the difference between HIV-I and HIV 2?
Two types of HIV are currently recognized: HIV- I and HIV-2. Worldwide, the predominant
virus is HIV- I. 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 initial infection and illness is longer in the
case of HIV-2.
How many subtypes do we know about?
We currently know of at least 1I0 genetically distinct subtypes of HIV I 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 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.
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.