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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.