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.
