Download Sexual HIV Transmission

The Past, Present, and
Probable Future of the
HIV/AIDS Pandemic
James Chin, MD, MPH
Clinical Professor of Epidemiology
School of Public Health
University of California, Berkeley
E-mail: [email protected]
Epidemiology – is the study of all factors that
may influence or determine the patterns and
prevalence of a disease or condition in
populations.
Public Health – “is the science and art of
preventing disease, prolonging life, and
promoting health and efficiency through
organized community effort …” [C.E.A.
Winslow – 1920]
Public Health Surveillance – is the systematic
collection, analysis, and dissemination of
all data that may be needed or relevant for the
prevention/control of a public health problem.
Jon Mann and senior staff of the Global Programme
on AIDS (GPA), World Health Organization (WHO),
Geneva, Switzerland, late 1987
Natural History of HIV Infection
• HIV infection is probably lifelong, and severe immune
deficiency will develop in up to half of HIV-infected
adults within 8 years after infection.
• Once severe immune damage develops, the infected
person is susceptible to many opportunistic infections
and cancers, and these illnesses are surrogate/indirect
indicators of the immunodeficiency due to HIV and
collectively they constitute the diagnosis of
Acquired ImmunoDeficiency Syndrome (AIDS).
• Annual progression rates from HIV infection to the
development of AIDS is similar in Haiti, Thailand,
Uganda, and “Western” countries (median of 8 years).
• Survival after the onset of AIDS is, in the absence of
anti-HIV treatment, short, and is usually less than
1 year in developing countries.
HIV Transmission
 HIV is transmitted from person to person primarily via
blood or other body fluids that may contain some blood.
 The risk of HIV transmission for any exposure/contact is
directly related to the amount of blood exchanged.
 The risk of transmitting HIV infection via blood
transfusion from an infected donor is close to 100%,
whereas the risk of an HIV-infected female transmitting
HIV to her sex partner can be as low as 1 per several
thousand episodes of vaginal intercourse.
 In the absence of facilitating factors, sexual HIV
transmission is several hundred times less infectious
compared to most other sexually transmitted agents.
 Anal intercourse is more efficient for HIV transmission
because of increased tissue trauma, but is still very low
compared to agents such as syphilis and gonorrhea.
Key Concepts and Factors Needed to
Fully Understand HIV Epidemiology
•
•
•
•
•
•
The reproductive number (Ro) of an infectious
disease agent
Epidemic (Ro >1) versus non-epidemic (Ro <1)
sexual HIV transmission
Low sexual (vaginal or anal) HIV transmission rates
in the absence of facilitating factors
Paramount importance of different patterns of sex
partner exchange (serial or concurrent) and marked
differences in the prevalence of such risky sex
behaviors within and between populations
HIV “bridges” from HIV risk groups to the “general
public” are usually bridges to nowhere
Poverty is NOT a major determinant of high HIV
prevalence rates
The Reproductive Number (Ro) of HIV
• The reproductive number (Ro), describes, in a single
value, the epidemic potential of an infectious agent
in a specific population.
• When, on average, one infected person infects more
than one other person, Ro is >1 and epidemic
spread will result. When, on average, one infected
person does not infect more than one other person,
Ro is <1 and epidemic spread does not occur.
Ro for HIV via sexual transmission is dependent on:
(1) probability that a sex partner is infected with HIV [p];
(2) probability of HIV transmission per coital act [r];
(3) number of unprotected coital acts with different sex
partners [n1, n2…]
Ro = (p x r x n1) + (p x r x n2)…
Understanding HIV/AIDS Numbers
Reported numbers of HIV infections or AIDS
cases are usually grossly under-reported!
Official numbers may be reported cases or may
be officially estimated cases.
Estimated numbers may be derived by official
expert groups or can be the estimated number of
an AIDS “expert” or an external agency.
Actual numbers or the real numbers represent
the “Holy Grail” for epidemiologists. They can, at
best, be estimated via an objective process using
the most reasonable assumptions and data
available.
How Reliable are HIV Prevalence Estimates?
• Estimation of HIV prevalence is more of an art than a
science. With the many uncertainties in HIV serologic
data and the limitations of the data, methods, and
assumptions used, estimation of HIV numbers cannot
be precise.
• Current HIV prevalence estimates tend to be high
because of insufficient data on urban/rural differentials.
Very high (>10% of the 15-49 year old population) HIV
prevalence estimates can be off by plus or minus 50%;
High (>1% & <10%) estimates can be off by 1-2 folds;
Moderate (>0.1% & <1%) prevalence estimates can
be off by several folds; and
Low (< 0.1%) prevalence estimates can be off by up to
10 folds!
Estimated HIV Prevalence - 2001
Country
15-49 population
No. HIV+
Ukraine
25,251,000
250,000
0.990
1/100
Russia
78,166,000
700,000
0.896
1/110
Greece
5,269,000
8,800
0.167
1/600
Armenia
2,152,000
2,400
0.112
1/900
118,163,000
120,000
0.102
1/1,000
Iran
37,396,000
20,000
0.053
1/1,900
Morocco
16,373,000
13,000
0.079
1/1,250
Jordon
2,561,000
<1,000
<0.039
<1/2,500
Georgia
2,726,000
900
0.033
1/3,000
Iraq
3,067,000
<1,000
0.033
1/3,000
Azerbaijan
4,529,000
1,400
0.031
1/3,250
Egypt
36,301,000
8,000
0.022
1/4,500
Bangladesh
72,340,000
13,000
0.018
1/5,600
3,915,000
400
0.010
1/10,000
36,857,000
3,700
0.010
1/10,000
8,481,000
87
0.001
1/100,000
Indonesia
Bulgaria
Turkey
Syria
%HIV+
Estimated HIV Prevalence in
10,000 Pregnant Females - 2002
Country
1
2
3
4
5
6
7
8
9
10
Botswana
South Africa
Haiti
Cambodia
Thailand
India
USA
Malaysia
Philippines
Turkey
Number
HIV Transmission
3,800
Primarily heterosexual
2,000
Primarily heterosexual
600
Primarily heterosexual
280
Primarily heterosexual
140 Mostly heterosexual & IDU
(1-300) 60
Focal heterosexual
(1-200) 25
Mostly MSM & IDU
3-4
Primarily IDU
1 No epidemic HIV spread
<1 No epidemic HIV spread
Major Public Health Question
Why is epidemic heterosexual HIV
transmission almost non-existent in most
heterosexual populations outside of subSaharan Africa?
Possible answers:
• Effective HIV prevention programs.
• Insufficient time for HIV to “bridge” into the
“general” population from current pockets
of HIV-infected persons.
• Insufficient sexual risk behaviors in most
“general” populations.
Facilitating* or Inhibitory Factors
for Sexual HIV Transmission
• Any factor that can cause lesions in the genital or
rectal epithelium (i.e., concurrent STI, especially
ulcerative STI such as genital herpes, “dry sex”,
traumatic sex, etc., etc.) can be a facilitating factor.
• New (Incident) HIV Infections are highly infectious.
• Male circumcision is associated with a reduced rate
of HIV transmission.
• Consistent condom use can prevent most sexual HIV
transmission (anal and/or vaginal).
• HIV subtypes C & E have not been correlated with
increased heterosexual HIV transmission.
• Poverty is not a major factor for high HIV prevalence
rates in MSM or SSA populations.
* Facilitating factors are not co-factors because they are not required for HIV transmission.
Poverty and HIV/AIDS
•
•
•
•
•
Countries with the highest HIV prevalence in sub-Saharan
Africa (SSA) are not the poorest countries in SSA and
most of the poorest countries in the world outside of SSA
have the lowest HIV prevalence.
Males and females in the highest socio-economic class in
SSA had the highest HIV prevalence because they had
very high numbers of different sex partners.
The highest HIV prevalence in SSA populations are in
female sex workers and the military who are not the most
impoverished groups in SSA, but they have the highest
sex partner exchange rates.
MSM with very high sex partner exchange rates had the
highest HIV prevalence whereas MSM who were mutually
monogamous had low or zero HIV prevalence.
Poverty was not and is not considered to be a significant
factor for high HIV prevalence in MSM populations.
Two Major Patterns of
Sexual HIV Transmission
1. Because of the low HIV infectivity via vaginal or anal
intercourse, epidemic (Ro >1) sexual HIV
transmission has only occurred where there are:
a high frequency of sex partner exchange and
mixing, i.e., having multiple and concurrent sex
partners; and a high prevalence of factors that can
facilitate sexual HIV transmission.
2. The non-epidemic (Ro <1) sexual pattern of HIV
transmission occurs from HIV-infected persons
(regardless of how they were infected) to their
regular sex partner(s), i.e.,”bridging”. Further HIV
spread from these regular partners (i.e., the
epidemic pattern) can only occur if these partners
have some HIV-risk behavior(s).
Sexual HIV Transmission
• Several datasets are available that quantify the
prevalence of sexually transmitted diseases
(STD), especially ulcerative STD, and sexual risk
behaviors in different populations. These data
show a general concordance between HIV
prevalence rates, STD prevalence rates, and the
pattern and prevalence of sexual risk behaviors.
• These data indicate that the pattern and
prevalence of sex partner exchanges and
facilitating factors for sexual HIV transmission in
many sub-Saharan African (SSA) populations,
are often 1-2 orders of magnitude greater
compared to most populations outside of SSA.
Major Determinants of Epidemic
Sexual HIV Transmission
1. The prevalence of persons who regularly
have multiple and concurrent sex partners.
2. The size and structure of sex networks and
the extent of mixing (or “intersections”)
between different networks
3. Prevalence of major facilitating factors such
as dry and traumatic sex, ulcerative STI, etc.
4. Low percent of male circumcision
5. Low percent of consistent condom use for
casual and commercial sex encounters.
Global HIV Transmission Dynamics
Transmission
Epidemic (Ro >1)
Injecting
drug
users
(IDU)
Epidemic HIV transmission in IDU have occurred in
over 100 populations globally and as of 2004 is the
most common mode of epidemic HIV transmission
in many Asian and Eastern European countries.
Blood or
blood
products
Thousands of hemophilia patients received HIVinfected blood products in the early 1980s.
Epidemic HIV transmission via faulty commercial
plasma collection occurred in Mexico in the mid1980s and in China up to about the mid-1990s.
Sexual
Heterosexual
Heterosexual HIV epidemics occur where there is a
high prevalence of persons with multiple, and
concurrent sex partners and a high prevalence of
factors that can greatly facilitate sexual HIV
transmission. Such epidemic transmission has
occurred primarily in sub-Saharan African, and to a
lesser extent in several Caribbean populations and
in a few Asian populations.
Epidemic HIV transmission in MSM has occurred
primarily in large cities in Western countries where
large MSM sex networks are present.
Men who
have sex
with men.
(MSM)
Non-epidemic (Ro <1)
All epidemic HIV
transmission is
invariably followed
by “non-epidemic”
sexual HIV
transmission to
regular sex
partners, most of
who do not have
any significant HIV
risk behaviors and
thus, there is no
further spread into
the “general
population
Where no epidemic HIV
transmission has
occurred, the majority
or a very large percent
of HIV infected persons
acquired their infection
outside of the country.
Prevention of Sexual
HIV Transmission
• Prevention of epidemic (Ro >1) sexual HIV
transmission requires:
elimination or modification of risk behaviors
such as having unprotected sex with multiple
and concurrent sex partners.
• Prevention of non-epidemic (Ro <1) sexual
HIV transmission requires:
identification of HIV-infected persons and
routine voluntary HIV testing and counseling
(VTC) for their regular sex partners.
Conclusions
The AIDS pandemic is the most severe
infectious disease pandemic since the
1918 “Spanish flu” pandemic.
However, the demographic impact of the
AIDS pandemic will be very uneven –
ranging from very severe in many subSaharan African populations, moderate to
minimal in several Caribbean and Asian
populations and not measurable in most
other populations.
Conclusions
The “Doomsday” type heterosexual HIV scenarios
for current low HIV prevalence countries are not
based on any sound epidemiology and have led to
inappropriate and ineffectual prevention strategies
directed more to the general public than to persons
with the highest HIV risk behaviors.
The wide differences in potentials for epidemic
sexual HIV transmission, based on differences in
the patterns and prevalence of sexual risk
behaviors, must be recognized, accepted, and
routinely monitored in order to design and focus
prevention strategies where they are most needed
and most effective.