SAFE SEX

Historically, the primary emphasis in health care has been the diagnosis and treatment of disease, but over the past two decades there has been a shift toward giving equal attention to lifestyle changes that maintain health and prevent disease.  The dominant health related lifestyle themes are related to diet, exercise, tobacco use, alcohol and drugs, preventing injury, and sexual safety, the subject of this Knol.

This Knol addresses safer sex as a lifestyle that can prevent infection with human immunodeficiency virus (HIV) and other sexually transmitted diseases (STD).  It is oriented to the individual who might be at risk, rather than to the broad prevention strategies that health departments or other organizations might use, such as widespread diagnostic screening for chlamydia or HIV.  First I address the scientific basis for transmission of infection, then discuss the ways sexually active persons can protect themselves and their partners.  Throughout this Knol, the term “pathogen” means any disease-causing (pathogenic) bacteria, virus, fungus, or parasite.

THE SCIENTIFIC BASIS OF STD/HIV TRANSMISSION

The scientific basis for STD transmission encompasses four domains. The first is biology, specifically the interactions between pathogen and host and, in the case of HIV, the amount of virus in the blood and genital secretions. The second domain involves conditions that can increase transmission efficiency, called cofactors. The third domain is human sexual behavior, which determines the frequency with which susceptible and infected persons interact in ways that allow transmission. The fourth and final domain includes the technologies available to reduce transmission, such as condoms and other barriers.

Biological Principles

Exposure.  Many people assume that “just one virus” introduced into a person’s body in the right way is sufficient for any infection to take hold.  In fact, there are major differences in transmission efficiency from one pathogen to another.  A single rabies virus introduced into the bloodstream or under the skin may be sufficient for an inevitably fatal outcome.  On the other hand, our bodies harbor billions of virulent staph, strep, and intestinal bacteria, yet for the most part we remain infection free because of natural defenses.  These defenses include physical barriers like the skin and mucous membranes (the linings of most body cavities); chemical defenses, such the natural acidity of the stomach and vagina; and immunological defenses, the antibodies and sensitized cells that are lethal to invading pathogens either through “natural” immunity or following sensitization by earlier exposure to the same or similar organisms. 

Immunity.  Infected persons become resistant after their first infections with some STD pathogens.  Persons infected with hepatitis B virus (HBV) are immune to new infection, and those previously infected with particular type of human papillomavirus (HPV), such as HPV-6, are immune to reinfection with the same type.  The immunity that develops to natural HBV or HPV infection has been exploited in developing effective vaccines.  Those infected with herpes simplex virus (HSV) type 1 or 2 are highly resistant, if not completely immune, to reinfection with the same HSV type.  Therefore, a person with recurrent oral herpes due to HSV-1 is unlikely to acquire genital herpes due to HSV-1; and couples who already share an HSV-2 infection are at no risk for back-and-forth (“ping-pong”) transmission.  Lesser immunity develops to other STDs.  For example, persons who recover from syphilis or chlamydial infection are somewhat less susceptible, but not immune to new infection.  No measurable immunity follows gonorrhea. 

Environment.  Sexually transmitted bacteria do not tolerate drying, do not survive in microscopic droplets, and have evolved for survival in the genitals or rectum.  Therefore transmission cannot occur through the air or through non-intimate personal contact, but instead requires the overt exchange of genital secretions or the direct contact of moist genital surfaces — that is, sex.  Some STD pathogens, in particular HIV and HBV, also circulate in the blood and can be transmitted by exposure to blood or by organ transplantation.  Transmission by routes other than sex occurs only through other “intimate” exposures, as when babies are infected in the uterus or during labor and delivery.  Transmission does not result from contact of infected secretions or blood with intact skin, by airborne exposure, or by shared toilet seats, moist towels, or intimate clothing.  The latter have been invoked through history as face-saving explanations, but they have no basis in fact.

Efficiency of Transmission.  Even under optimal conditions for infection, transmission of most STDs is relatively inefficient.  For example, only about 20% of men become infected for each episode of unprotected vaginal intercourse with an infected partner.  The transmission efficiency in the opposite direction is higher, around 50%, because infected secretions are deposited in the vagina and have direct access to the cervix.  The efficiency of chlamydia transmission is similar to that of gonorrhea.   Calculations for transmission of herpes simplex virus type 2 (HSV-2), the main cause of genital herpes, suggest that most exposures do not result in transmission, even when the virus is present in the infected partner’s genital area.  In contrast, human papillomavirus (HPV) may be transmitted with something approaching 100% efficiency.   

HIV is among the least easily transmitted STD pathogens.  The most efficient transmission practice for HIV, unprotected penile-anal intercourse, probably results in one transmission from the insertive to the receptive partner for every 100 exposures, and the transmission risk in the opposite direction is around one in a few hundred exposures.  Partly for these reasons, sexually transmitted HIV infections occur dominantly in people whose sexual practices and lifestyles entail repeated high risk exposures.  Relatively few persons become infected after only one exposure. 

Viral Load.  Another biological consideration, for HIV, is the amount of HIV in the blood, known as viral load, which in turn influences virus levels in semen and cervical fluid.  Viral load varies during the course of a person’s HIV infection, with treatment for HIV, and in the presence or absence of other infections.  The highest viral load is observed in the first several weeks after catching HIV. The viral load then declines as the immune system partially controls the infection.  Over the next few years, viral load is relatively low, although it varies widely from one person to another.  (This variability partly explains differences in the time to death from AIDS, with a few people dying in a year or two and others surviving over ten years, even without treatment.)  People who take drugs to combat HIV have lower viral loads.  Viral load typically climbs to high levels late in the disease, as death approaches.  Over the intervening years, illnesses like malaria, influenza, pneumonia, and other STDs—almost any infection—can cause the viral load to rise.   

For these reasons, the overall risk of HIV transmission in a particular community or country depends in part on how recently the infection was introduced into the population (leading to more people with early infection and high viral loads), the frequency of other infectious diseases in the population, the proportion of infected persons receiving antiviral therapy, and the number of people with advanced AIDS who remain sexually active.  Differences in these factors largely explain the wide variation in HIV sexual transmission rates in North America and Western Europe compared with areas like southern Africa.

HIV Transmission Cofactors

Transmission cofactors are conditions that enhance the likelihood of transmitting HIV, beyond the sexual exposure itself. 

STD and other Genital Infections.  Some of the earliest studies of HIV transmission indicated that genital ulceration—i.e., open sores—increases the chance of HIV transmission.  It was initially thought that the mechanism was simply that the break in the skin or mucous membrane allowed access of virus to the bloodstream, but that is a small part of the explanation.  It is now understood that any condition causing inflammation of genital or rectal tissues increases transmission from HIV infected persons, and also increase susceptibility to the virus in persons exposed to HIV.  Genital herpes due to HSV-2 is the single most important STD in enhancing HIV infectiousness and susceptibility, for reasons of biology and because HSV-2 infection is typically the most prevalent STD in sexually active populations.  Persons with HSV-2 infection have up to four times the risk of acquiring HIV, if exposed, than persons without HSV-2, even if they have no symptomatic genital herpes outbreaks.  Similarly, HSV-2 infected persons are more efficient HIV transmitters than those without herpes.  Gonorrhea, chlamydia, syphilis, chancroid, and trichomonas infection in women all have similar effects.  HPV appears to have little influence on HIV transmission, because HPV infections cause little if any inflammation. 

The Foreskin and Male Circumcision.  Early in the AIDS epidemic, epidemiological research showed a geographic association of HIV rates, especially in developing areas like sub-Saharan Africa.  Where uncircumcised men dominated the population, HIV was more common; where circumcision was the rule, HIV infection rates were lower.  Research soon began to show the reasons:  the lining of the foreskin and the surface of the penis in uncircumcised men is thinner than in men without foreskins, and it contains large numbers of cells that are especially susceptible to HIV if exposed.  In addition, the intact foreskin provides a reservoir that traps a partner’s genital secretions and keeps them in contact with the susceptible tissues.   

Final proof of the importance of circumcision came from three randomized, controlled research trials in which men volunteered to be circumcised immediately or after delaying for several months.  In all three studies, the men who remained uncircumcised had double the rate of new HIV infections as those who were circumcised, despite having similar numbers of sex partners.  In response to these research findings, plans are now underway for widespread male circumcision in some developing countries.  However, the importance of circumcision in North America and Europe is uncertain, owing to important differences in the epidemiology and transmission risks of HIV.  Where heterosexual transmission accounts for the bulk of HIV infections in a population, circumcision may have great import.  However, it remains to be seen whether routine circumcision, either of infants or sexually active men, might be an effective HIV prevention strategy outside developing countries with large heterosexual HIV epidemics.  Certainly sexually active men in North America or Europe should not now seek circumcision in order to prevent HIV or other STDs.

Sexual Practices.  Anal intercourse is the most efficient sexual transmission mechanism for HIV.  The lining of the rectum is only one cell layer thick, compared to 50 or more cell layers in the membranes that line the mouth and vagina.  Second, anal sex is inherently more traumatic than vaginal or oral sex, increasing the risk of injury and exposure of HIV to the bloodstream.  Oral sex carries much lower risk for transmission of all STDs, including HIV, than vaginal or anal intercourse.   

Gonorrhea often infects the throat, but infection of the throat is less readily transmitted than genital infection.  Chlamydia rarely infects the throat and there is no measurable risk of transmission from the oral to the genital partner or vice versa, despite common beliefs to the contrary.  Nongonococcal urethritis (NGU) can be acquired by fellatio, and sometimes might be caused by otherwise normal oral bacteria, perhaps explaining why NGU sometimes appears in monogamous couples.  In recent years, almost half of all newly acquired cases of genital herpes are acquired by oral sex and are caused by HSV-1, the cause of recurrent oral herpes.  On the other hand, genital HSV-2 infection is uncommonly transmitted to the oral partner.

Sexual Behavior

In some ways, the behavioral aspects of STD transmission seem straightforward.  Sex with an infected partner risks transmission and sex with an uninfected partner does not. Unfortunately, judging whether a potential partner might be infected isn’t so simple.  The sociological aspects of sexual behavior—the patterns of sexual behavior in the community—largely determine the likelihood that a particular person might be infected.  The number of partners is an obvious factor, and other things being equal, increased numbers of partners and increased frequency of partner change increase the likelihood of infection. That said, numbers of lifetime or recent sex partners do not always predict STD rates.  For example, some persons change partners frequently for many years without acquiring known STDs (although many are asymptomatically infected with HPV or HSV), whereas others have few partners and are infected repeatedly.  Even commercial sex work carries an inconsistent relationship to STD risk.  Finally, all STDs are substantially more common in African Americans than in other race/ethnicity groups in the United States, yet the average number of sex partners per person is not appreciably greater than in whites. 

Recent research implicates societal patterns of sexual behavior as key determinants of STD frequency.  Population groups can be viewed as comprising sex partner networks.  Whereas the average numbers of sex partners among persons in a partner network has little correlation with STD rates, the variation in numbers between individuals has a substantial influence.  In other words, the presence of a small minority of persons with multiple partnerships disproportionately raises the risk for all members of the network. Concurrency reflects the frequency with which persons in a population have overlapping partnerships.  The more persons in a network who start new sexual relationships before terminating previous ones, the more STD rates rise, even with no change in the average number of sex partners per person or the rate of partner change. 

Prevention Technologies

Condoms and other Barriers.  Condoms in one form or another have been use for hundreds if not thousands of years for contraception and to prevent STD, but research on the effectiveness of condoms for STD prevention was largely lacking until late in the twentieth century.  The diaphragm and cervical cap, which protect the cervix but no other genital surfaces from exposure to semen, are designed primarily as contraceptive devices, but might provide slight protection against some STDs.  Because of fears that oral sex might transmit HIV, barriers such as dental dams (latex sheets used during dental procedures) and plastic wrap began to be recommended in the 1980s.  The female condom, inserted into the vagina and held in place by a flexible ring (not unlike the rim of a diaphragm) in theory might provide STD protection superior to that of the male condom, because the device provides a more extended barrier to genital skin contact between partners.  However, few data on STD prevention exist for any barrier except for the male condom. 

The mechanical effectiveness of condoms and other barrier devices is not in doubt.  Latex and polyurethane in the thickness used for condoms and other barrier devices do not permit transmission of any pathogen.  Natural membrane condoms, typically made of intestinal linings of sheep or other domestic animals, have pores that exceed the size of some pathogens, and penetration of viruses can be demonstrated in the laboratory.  However, the physical characteristics of fluids (e.g., viscosity and surface tension) probably prevent passage in amounts sufficient for disease transmission.   

The “use effectiveness” of condoms is different than biological efficacy, and accounts for human failures in proper use.  Consider the effectiveness of condoms for pregnancy prevention:  the efficacy of properly used condoms that do not break is nearly 100% (biological efficacy), yet couples who depend on condoms for contraception have only a 90% reduction  in pregnancy rates (use effectiveness), because condoms are not always used properly and sometimes rupture.  Moreover, research subjects often do not acknowledge failure in correct use – and studying condom efficacy also has been affected by “recall bias”.  Memories are fallible, and retrospective recall—whether condoms were used in the past, how frequently, whether proper technique was employed, and with which partners—is notoriously unreliable.  Finally, the most frequent condom users often are people at highest risk for STD, leading to the apparently contradictory results of some studies showing higher infection rates in condom users than non-users. 

For all these reasons, several research studies seemed to show little protection against some STDs, especially HSV and HPV infection, for which transmission depends on skin-to-skin contact more than secretion exchange.  These research results opened  a window for political opportunists to claim that condoms are ineffective in preventing STDs.  More recent research, however, has demonstrated high efficacy for condoms in preventing all STDs. These recent studies limited recall bias by asking study participants to document sexual events with and without condoms in real time diaries and to report the data soon afterward.  Using this approach, it is was demonstrated that sexually active young women whose partners used condoms 100% of the time had a 90% reduction in new HPV infections.  However, anything less than 100% use markedly reduced the effectiveness of condoms, and the rate of HPV transmission was not reduced at all in women who reported using condoms 70% of the time or less. 

Taken collectively, the accumulated data suggest nearly 100% biological efficacy of condoms against infections transmitted by secretions or blood, such as gonorrhea, chlamydia, or HIV, and lesser but nevertheless useful effectiveness in preventing infections transmitted by skin-to-skin contact, such as HSV, HPV, and syphilis.  In other words, when condoms fail, the problem is primarily in use effectiveness.  Some persons with political or religious agendas contend that condoms should not be advised because they are not infallible.  However, 100% efficacy is not a criterion for recommending any safety device.  Seat belts prevent injury and death, even though some people die in vehicular accidents while using them. 

Microbicides.  Microbicides are chemicals that inhibit or kill pathogens.  The differences compared with antibiotics are that most microbicidal compounds do not have biological origins, as antibiotics do; and microbicides are intended for local use in the vagina or rectum, not for systemic administration.  In other words, microbicides are more like antiseptics.  Intense research is in progress to identify microbicdes that are safe and easy to use, especially by women, preferably without their male partners’ knowledge.  The first microbicide subjected to rigorous research was nonoxynol-9, the active ingredient in most spermicides used for contraception.  Nonoxynol-9 kills many bacteria and viruses and was widely considered both safe and promising as a microbicide.  Unfortunately, clinical trials revealed an increased risk of HIV, owing to damage to the vaginal lining that increased the infectivity of HIV.  Since those disappointing early results, a large number of other compounds have been in clinical trials, so far with largely negative results.  Although the ultimate promise of microbicides is great, no currently available products noticeably reduce the risk of HIV or other STDs.

PREVENTING HIV AND STD IN THE REAL WORLD

How does all this translate into information for persons who seek to avoid HIV and other STDs?  The following information reflects the author’s personal perspective, informed primarily by the available scientific data and secondarily by the standard advice from numerous health agencies, such as the Centers for Disease Control and Prevention [link] and the World Health Organization [link], as well as three decades of practical experience advising patients.  

Abstinence and Monogamy

It seems obvious that a sexually inactive person is not at risk for acquiring an STD.  However, the line between sexual activity and abstinence is a blurry one.  Masturbation, body rubbing, kissing, other oral contact (e.g., mouth to breast), hand-genital (or hand-anal) contact, and oral-genital contact all are sexual activities.  For practical purposes, however, STD/HIV transmission occurs only as a consequence of penile-vaginal or penile-anal intercourse or though oral-genital or oral-anal contact (fellatio, cunnilingus, analingus).  Shared sex toys and other devices – and sometimes hand-genital or -anal exposure -- permit occasional STD transmission, especially in lesbian couples and men who have sex with men (MSM), and rare cases of genital herpes or HPV infection might be acquired by hand-genital contact.  But “sexually active” is used here to describe people who participate in insertive penile-vaginal or penile-anal intercourse, fellatio, cunnilingus, or analingus.  

Attempts to be abstinent are often unsuccessful, and they may even be counterproductive.  Young persons who take pledges of abstinence until marriage typically delay the age of first sex by an average of nine months, from an average of 15.5 years of age to slightly over 16 years.  Further, abstinence pledgers apparently have an elevated risk of STD or unwanted pregnancy when sexual activity begins.  This seeming paradox probably is explained by failure to have condoms handy and to use them, owing to the inherent conflict between sexual intent and keeping condoms available.  Permanent mutual monogamy is completely effective in preventing new STD, but even monogamy is no guarantee against STD if either partner previously had other partners.  For example, up to a quarter of all persons acquire genital HPV infections from their very first sex partner and over half are infected by one of their first three partners.  As a result, abnormal pap smears resulting from HPV infection are almost as frequent in permanently monogamous women as in women with several lifetime partners.  

Selecting Sex Partners

Meeting Partners.  The main dilemma in STD/HIV prevention for sexually active persons is in identifying partners at low risk for current infection.  Other aspects of prevention—deciding on specific sexual practices or to use a condom—may not be easy, but they are straightforward yes/no decisions.  By contrast, the likelihood a particular partner is infected lies on a continuum.  Some partners are inherently and obviously risky, including many (but not all) commercial sex workers, persons who recently emigrated from countries with high HIV rates, injection drug users, and MSM with many sex partners.  Sex between persons within hours of meeting each other, especially in contexts of alcohol or drug use, is especially high risk for STD transmission.  These behaviors often denote a risky lifestyle, poor judgment, previous similar encounters, and inconsistent condom use.  (Such partnerships typically confer higher STD risk than commercial sex.)  Some venues in themselves connote high risk for STD and HIV, such as bath houses attended by MSM expressly for sex.   

On the other hand, partners who meet in the workplace or are introduced by friends in a social context are less likely to be infected.  The lowest risk may pertain when both partners are “in the market” for a committed relationship and socialize before having sex.  Some such couples choose to be tested for common STDs before becoming sexually active.  This is not a bad thing, but has little effect at a societal level, because such persons are exactly the sort least likely to be currently infected.  Internet dating carries both risks and opportunities for sexual safety.  Short term liaisons negotiated on line clearly risk selecting partners at high risk, but this approach also offers relatively impersonal opportunities to share information about sexual lifestyle, intentions for condom use, and infection status. 

Do Ask, Do Tell.  In all circumstances, a powerful protective strategy is simply to ask a prospective partner whether he or she is infected or at risk.  While some people are untruthful, especially in response to a hurried query at the height of arousal, and others are infected unknowingly, most persons do not lie when asked directly about HIV, herpes, or other STDs.   Asking about infection status is particularly important for MSM and for other persons at high risk for HIV infection.  In North America and Western Europe, on average 5-10% of MSM are infected with HIV and most know of their infections.  In my opinion, no man intending sex with another man ever should proceed without first asking about his partner’s HIV status, sharing his own, and foregoing sex or recommitting to condom use when one partner but not the other is infected.  This pertains even when safer sex (e.g., oral sex, mutual masturbation) is planned, because condoms break and intentions for safer sex commonly are forgotten at the peak of passion.  The same advice is valid for other settings with high HIV risk and theoretically this approach makes sense for all sexually active populations and for preventing STDs other than HIV.  However, the lethality and high rates of HIV raise the stakes for MSM in particular. 

Sex Between Infected Partners.  Uncertainty exists about the safety of sex between partners already infected with HIV.  It is not known whether infection with a second HIV strain brings increased risk of accelerated progression to AIDS.  Further, partners who both are infected with HIV may still transmit syphilis, herpes, or other STDs, which in turn might adversely affect their HIV disease.  However, on balance, sex between mutually infected partners is inherently safer than sex between infected and uninfected persons.  Probably little harm comes from sex between persons who share the same strains of HSV, HPV, chlamydia, gonorrhea, or other STDs.

Sexual Practices

Penile-vaginal and penile-anal sex account for the vast majority of STD transmission events.  Based on exposure histories given by HIV-infected persons, the risk of transmission by fellatio, if one partner is infected, has been calculated at one transmission for every 10,000 exposure for an oral partner exposed to an infected person’s penis, and one in 20,000 in the opposite direction.  (The latter figure is equivalent to receiving oral sex once daily from infected partners for 55 years.)  These rates are roughly ten-fold and 100-fold less efficient than the calculated transmission rates by vaginal or anal intercourse, respectively.  Few if any HIV transmissions by cunnilingus have been documented, notwithstanding occasional claims by infected persons. 

These numbers should be interpreted with many grains of salt.  Such rates clearly are too low to sustain HIV infection in most populations, with the possible exception of anal sex between MSM, and such low transmission risks cannot be true for all populations at all times.  The cofactors that raise the transmission efficiency are cited above, and collectively account for widely differing HIV transmission rates in various populations.  Nevertheless, two conclusions are valid for sexually active persons in North American and Europe:  in general, oral sex is safe sex; and even for anal sex or vaginal sex with partners at high risk for HIV, most individual exposure events do not result in HIV transmission.  This does not imply that sexually active persons should be cavalier about precautions against a fatal STD, but neither is panic warranted after most risky encounters. 

Oral sex also is considerably less risky than vaginal or anal sex for STDs other than HIV.  For practical purposes, only four STDs are acquired by the genital partner in fellatio or cunnilingus:  herpes due to HSV-1, urethral gonorrhea in men (but probably not genital gonorrhea in women), NGU in men, and syphilis, which is rare.  For the oral partner there is substantial risk for gonorrhea but low risk for most other STDS.  The frequency with which oral sex results in oral HPV infection is uncertain but apparently uncommon.  Further, when transmission occurs, overt oral warts or other symptoms are rare.  The frequency of head and neck cancers, often associated with high risk genital HPV types, is rising, raising the possibility of an association with the increased frequency of oral sex.  However, head and neck cancers are uncommon, with around 30,000 cases per year in the United States, and only half to two thirds of these are associated with HPV.  

Condoms and Other Barriers

The male condom will remain a mainstay of STD/HIV prevention for the foreseeable future, probably forever.  Female condoms probably protect better against skin contact compared with the male condom but have not gained broad acceptance.  Male condoms are highly protective against transmission of all STD pathogens, but are most effective in preventing the infections transmitted through secretions and blood, like HIV, chlamydia, and gonorrhea, and are somewhat less effective against HSV, HPV, and syphilis, which are transmitted primarily by skin to skin contact.  Latex or polyurethane condoms are universally recommended over natural membrane condoms, but the latter probably provide substantial protection.  Certainly persons who prefer natural membrane condoms should use them in preference to avoiding condoms altogether.  Oil-based lubricants may weaken latex and increase the risk of condom rupture.  However, there is no evidence that oil facilitates transport of pathogens across an intact latex membrane, and protection probably is not reduced if an oil based lubricant is used inadvertently and the condom remains intact.  For the reasons discussed earlier, spermicidal creams containing nonoxynol-9 should not be used to prevent HIV and other STDs, and condoms packaged with nonoxynol-9 should be avoided except for contraception. 

The use of condoms for fellatio is controversial.  Consistent use would prevent both partners from virtually all STDs transmitted by oral sex, but the risk is low without condoms and many sexually active persons find them distasteful, literally and functionally.  There is similar antipathy to dental dams for cunnilingus or analingus, and most health experts recommend routine use of barriers for oral sex half-heartedly if at all.

Immunization

Vaccination against hepatitis B is now a routine component of childhood immunization in the United States, but until that policy results in a largely immune adult population, persons at risk for STDs should request the HBV vaccine.   HBV vaccination should be routine in MSM, who have the highest rates of sexually transmitted HBV.  The currently marketed HPV vaccine (Gardasil®, Merck) is highly effective against HPV-6 and 11, the dominant causes of genital warts, and HPV-16 and 18, which cause about 70% of cervical cancer.  A second product (Cervarix®, GlaxoSmithKline) soon will be approved for use in the United States, but it protects only against HPV-16 and -18.  Either vaccine is recommended for routine use in sexually active young women up to age 26, as well as in teen and pre-teen girls.  Both of the HPV vaccines undoubtedly will prove effective in older women and in men, but the importance of expanding immunization to men or older women is uncertain.  Future HPV vaccines may offer protection against additional HPV types and recommendations may change. In preliminary trials of a vaccine against HSV-2, women but not men were partially protected against infection.  Expanded studies are underway, but the prospects for an important role of this particular vaccine to prevent genital herpes are uncertain.  Effective immunization against HIV, gonorrhea, chlamydia, or syphilis remains a distant hope.

Treatment to Prevent Transmission

Treatment of the curable bacterial STDs (gonorrhea, chlamydia, syphilis) is a mainstay of prevention.  Antiviral therapy does not cure HSV infection, but the risk of transmission of HSV-2 is reduced at least by 50% by suppressive treatment of infected persons with valacyclovir (Valtrex®, GlaxoSmithKline).  HIV infected patients who take antiviral therapy have lowered viral loads in blood, semen, and female genital secretions, and are less likely to infect their sex partners.  The risk of HPV transmission probably is reduced after warts or cervical lesions are ablated.  Preventive treatment of exposed persons also has an important role in prevention, and antibiotic treatment is routinely given to the partners of persons with gonorrhea, chlamydia or syphilis, whether or not they have documented infection. 

Treating STDs reduces HIV infectiousness, and treating gonorrhea reduces the amount of HIV in infected men’s semen.  In theory, therefore, STD treatment might be an important HIV prevention strategy.  However, trials of STD treatment to prevent HIV transmission have been disappointing.  One study in Tanzania demonstrated benefit, but treatment of entire villages in Uganda for common STDs had no appreciable effect on HIV transmission.  More recently, an international multicenter trial found no reduction in new HIV infections in HSV-2 infected persons treated for herpes.

Post-Exposure and Pre-Exposure Prophylaxis of HIV

Administration of antiretroviral drugs—therapy active against HIV—within 72 hours of exposure to prevent HIV in exposed persons is called post-exposure prophylaxis.  PEP often is successful in preventing infection in health care personnel exposed to the virus through injuries with HIV-contaminated sharp instruments.  Although documentation of efficacy following sexual exposure is less clear, PEP often is administered after high risk sexual encounters.  Because the drugs carry their own risks and treatment must be taken for several weeks, PEP generally is given only after particularly high risk exposures, such vaginal or anal sex with a partner known or suspected to be infected with HIV.  Pre-exposure prophylaxis (PrEP) is treatment before exposure, rather than afterward.  Animals can be protected by pre-treating them with antiviral drugs before exposing them to HIV-like viruses, but human efficacy trials of PrEP have not been completed.  Even if effective, widespread use might foster the selection and spread of HIV variants resistant to the drugs, reducing the efficacy of treatment for all infected persons.  Nevertheless, considerable underground use of PrEP probably is going on prior to known high-risk exposures between persons at high risk, especially MSM.

Other Prevention Strategies

Douching.  Vaginal douching has long been used by women in attempts to prevent STD.  However, douching probably is ineffective, and it is associated with elevated risk of bacterial vaginosis, pelvic inflammatory disease, and perhaps female infertility and tubal pregnancy.  Douching should not be used by women at risk nor should it be recommended by health care providers.

Washing and Urinating.  Washing with soap and water after sex is an intuitive prevention strategy of unknown effectiveness.  It probably is harmless, although a recent study reported the surprising finding of an increased risk of HIV infection in men who washed after sex.  Many men urinate after sex in hope of reducing STD risk.  No data support the practice, but it is unlikely to be harmful.

SUGGESTED READING AND INTERNET RESOURCES

1. Aral SO, et al.  Multilevel Approaches to STD Epidemiology and Prevention.  Chapter 89 in Holmes KK, et al (eds), Sexually Transmitted Diseases, 4th edition.  New York, McGraw-Hill, 2008:1753-65.  A summary of the scientific aspects of STD/HIV prevention.

2. Steiner MJ, et al.  Condoms and Other Barrier Methods for Prevention of STD/HIV Infection and Pregnancy.  Chapter 93 in Holmes KK, et al (eds), Sexually Transmitted Diseases, 4th edition.  New York, McGraw-Hill, 2008:1821-9.  A comprehensive summary of the topic, with review of the available scientific data on effectiveness of condoms and other barriers.

3. Aral SO, Douglas JM Jr, Lipschutz JA (eds).  Behavioral Interventions for Prevention and Control of Sexually Transmitted Diseases.  New York, Springer, 2007 (575 pp).  The first and only textbook devoted entirely to STD/HIV prevention; intended for health professionals but comprehensible to all readers.

4. Centers for Disease Control and Prevention, Divisions of STD Prevention and HIV/AIDS Prevention.  http://www.cdc.gov/sexualhealth/  Comprehensive information about all STDs and HIV and prevention strategies, from the nation's public health agency.

5. American Social Health Association.  http://www.ashastd.org  ASHA is the nation's premier nonprofit agency in support of STD prevention and offers comprehensive prevention materials and information about all STDs, including personalized telephone counseling.

6. Kohler PK, et al.  Abstinence-only and comprehensive sex education and the initiation of sexual activity and teen pregnancy.  Journal of Adolescent Health 2008;42:344-51.  A systematic review of the comprehensive Adolescent Health (AdHealth) database, showing that teens who experience comprehensive sex education are less likely than those exposed to abstinence-only programs to become sexually active or become pregnant.  Self-reported STD rates were comparable in both groups.