The process of HIV transmission
- HIV can survive outside the human body for only a brief period of time and, thus, cannot be passed on through the external environment.
- In order for HIV to be transmitted, direct contact of certain body fluids of an HIV-positive individual (blood, sperm, pre-seminal fluid, vaginal and anal fluids) with certain body points of an HIV-negative person (mucus membranes or open wounds) is required.
- HIV transmission from an HIV-positive mother to her fetus or newborn baby during pregnancy, delivery or breast-feeding (if no treatment is provided) is a distinct mode of transmission.
- HIV transmission can be prevented if prophylactic antiretroviral treatment (PEP) is administered immediately after a possible exposure to the virus.
From the moment it settles into the human body, HIV is an especially resistant virus and its elimination, at least through the currently available methods, is impossible. This is not the case, however, when HIV is located outside the human body. Exposed to the external environment (e.g. blood on the ground or other surfaces), HIV cannot survive for long. Its survival time though cannot be estimated accurately, as it depends on many and variable environmental factors, such as temperature, humidity, sun exposure and acidity (pH) . Relevant laboratory studies have shown that HIV can survive for only a few minutes (e.g. if exposed to high temperature, drought and direct sunlight) or up to a few weeks (e.g. if protected inside a syringe in a dark and cool place) . However, until today, there has been no case of HIV transmission from the external environment. In contrast, other viruses (such as influenza or hepatitis B and C) are much more resistant and remain contagious for a much longer period of time.
Consequently, in order for HIV to be transmitted from a HIV-positive to an HIV-negative person, a sufficient amount of viral particles should be transferred directly (through direct contact) from the body of the first to the body of the second. Therefore, a means of transfer, with sufficient amount of viral load from the body of the HIV-positive person, is on the one hand necessary and on the other hand, an access point for the virus into the body of the seronegative person.
Means of virus transmission can be specific body fluids of the HIV-positive person, in particular semen and pre-seminal fluids (in the case of men), vaginal fluids and breast milk (in the case of women), rectal fluids and blood. There are no infectious amounts of HIV in any other kind of body secretion (e.g. saliva, sweat, tears, urine, feces), unless it is mixed with enough blood quantity .
The possible access points for the virus are also specific: either exposed parts of the circulatory system (the arteries, the veins and vessels, where the blood circulates) or the mucous membranes that cover the cavities and internal passages of the body, primarily the genital and rectal mucosa (very rarely the oral mucosa and extremely rarely the ocular one). At these points there are certain types of immune cells that HIV can infect and, thus, begin its replication process. It is impossible for HIV to enter the body from any other part that is covered by skin, unless there are open wounds (i.e. access to the circulatory system) .
Regarding the entry of the virus to the bloodstream, it can occur either through direct blood transfusion (e.g. when sharing injection syringes for injectable drug delivery), through injuring by sharp objects that bear fresh blood (e.g. needles, razor blades, toothbrushes etc.), or through the direct contact of another infectious body fluid with an open wound (e.g. violent vaginal or rectal intercourse without the use of condom, oral sex when there are wounds inside the oral cavity). As for the entry of the virus through the preputial, vaginal and rectal mucosa, it can occur during sexual intercourse (rectal or vaginal) without the use of condom (see below Routes of transmission and non-transmission of HIV).
A specific and not fully understood case is the vertical transmission of HIV, i.e. from the mother to the fetus or infant, unless the appropriate protective measures are taken. The transmission can occur either during pregnancy, especially if placental injury occurs and the mother’s blood passes into the fetal circulation or during breastfeeding, as HIV that is contained in the breast milk can penetrate the baby’s immature digestive system. More commonly it occurs during childbirth, when the fetus comes into excessive contact with the mother’s blood and maternal secretions . However, due to the available therapeutic methods, vertical transmission is currently less than 1% possible .
Apart from the entry route, studies have shown that the likelihood of HIV transmission is affected by various other factors (see below Factors affecting the probability of HIV sexual transmission), the most important of which is the viral load of the HIV-positive person: the higher the viral load, the higher is the probability and vice versa.
Regardless of how HIV enters the human body, it is eventually transferred to the nearest lymph nodes in approximately 3 to 5 days .The lymph nodes are particularly important structures of the immune system and are found in many parts of the human body. Their main function is the control of body fluids regarding the presence of microbes and cancer cells. Also, a large number of lymphocytes is concentrated in the lymph nodes, which works as defense against pathogenic microorganisms. Among these, a large number of T4 helper lymphocytes (or CD4+ cells) can be found in the lymph nodes, which are the main target of HIV (see How does HIV affects the immune system?). So, from the moment the HIV particles reach the lymph nodes, they invade the adjacent T4 helper cells and the rapid multiplication process of the virus immediately begins. At this point, HIV has been settled into the human body forever.
The transfer of HIV from the initial site of infection to the lymph nodes can be prevented through the immediate administration of certain antiretroviral medication, usually referred to as PEP (Post-Exposure Prophylaxis). In this way, if someone thinks he/she has somehow been exposed to HIV, he/she should then contact the AIDS Referral Centre or Emergency Hospital Department of his/her area as soon as possible. There, a specialist will assess the likelihood of transmission and, if necessary, provide a prophylactic treatment, that can prevent the virus from settling into the body. In order for PEP to be effective, it should be provided as soon as possible, after the possible exposure (ideally 2-4 hours later). If 72 hours have passed, it is believed that HIV has already infected several CD4+ cells in the lymph nodes and PEP can no longer have any protective effect (see Post-exposure prophylaxis (PEP)) .
Transmission and non-transmission of HIV
In order to define the ways in which HIV transmission is possible or not as well as the likelihood of transmission in each case, two parameters should be considered. The first is whether the transmission in a specific way is theoretically possible, based on our scientific knowledge and understanding of the biological mechanisms of the virus and the physiology of the human body. The second parameter relates to the frequency at which transmission in a specific way actually occurs in reality, based on epidemiological studies and the statistical calculation of the respective probability. Thus, we can distinguish three categories of HIV transmission:
(a) Biologically possible and statistically frequent transmission modes
(b) Biologically possible but statistically rare transmission modes
(c) Biologically and statistically impossible transmission modes
Table 1 presents the estimates of the most recent relevant study  regarding the per-act likelihood of HIV transmission, depending on the type of exposure. So, as a hypothetical example, if 10,000 women have vaginal sexual intercourse without a condom with a HIV-positive partner, it is expected that the virus will be transmitted to 8 of them (confidence interval: 6-11). If the intercourse is though rectal, HIV is expected to be transmitted to 138 (102-186). However, these estimates do not consider the significant effect of factors that may increase (e.g. another STD present, high viral load) or reduce (e.g. circumcision, antiretroviral treatment or PrEP) the probability of transmission. Thus, the estimation of transmission probability for a specific, actual case of HIV exposure can be only approximate and the following estimates should be considered only as indicative.
|Type of exposure||Transmission probability per 10,000 exposures (95% CI)|
|Transfusion of HIV+ blood||9,250 (8,900 – 9,610)|
|Syringe sharing for injectable drug use||63 (41 – 92)|
|Needlestick injury||23 (0 – 46)|
|Receptive anal sex||138 (102 – 186)|
|Insertive anal sex||11 (4 – 28)|
|Receptive vaginal sex||8 (6 – 11)|
|Insertive vaginal sex||4 (1 – 14)|
|Receptive oral sex||(0 – 4)|
|Insertive oral sex||(0 – 4)|
|From mother to fetus/infant||2,260 (1,700 – 2,900)|
Common transmission routes
The most frequent ways of HIV transmission are:
- vaginal or anal sexual intercourse without the use of a condom
- sharing syringes and other works for injecting drug use
Various factors affect the likelihood of HIV transmission, like being the passive sexual partner, high viral load, presence of other STIs etc.
Anal sexual intercourse without a condom
Anal sexual intercourse (homosexual or heterosexual) involves the highest probability of HIV sexual transmission . During anal intercourse, the virus is transferred through the rectal or penile fluids and infects specific types of immune cells found in the respective mucous membranes. In addition, HIV can pass directly into the bloodstream, if scratches and wounds have occurred on the skin or mucosa of the area (e.g. due to violent intercourse or some other sexually transmitted disease).
HIV is more likely to be transferred from the insertive to the receptive partner rather than the opposite, especially if the insertive partner ejaculates inside the anus of the receptive one, just because the rectal mucosa is much larger, thinner and more susceptible to injuries . This certainly doesn’t mean that HIV cannot also be transmitted from the receptive to the insertive partner.
Other factors that have been found to affect the likelihood of HIV transmission during rectal sexual intercourse are the HIV-positive partner’s level of viral load, the presence of other sexually transmitted diseases, circumcision and the use of poppers (see below Factors affecting the probability of HIV sexual transmission).
Additional information in the “Preventing HIV sexual transmission” section
Vaginal Sexual intercourse without a condom
Condomless vaginal intercourse involves the second highest probability of HIV sexual transmission, but is worldwide the most common way of transmission. During vaginal sexual intercourse, the virus is transferred through the vaginal and penile fluids and affects specific types of immune cells located in the respective mucous membranes. In addition, HIV can pass directly into the bloodstream, if scratches or wounds have occurred on the skin or mucosa of the area (e.g. due to violent intercourse or some other sexually transmitted disease).
Concerning vaginal contact, the transmission probability is also higher for the receptive partner compared to the insertive one, especially if the man ejaculates inside the woman’s vagina, simply because the exposed vaginal mucosa is much larger and more susceptible to injuries. This by no way means that the opposite, the transmission from woman to man, cannot happen .
Other factors that have been found to affect the likelihood of HIV transmission during vaginal sexual intercourse are the HIV-positive partner’s level of viral load, the presence of other sexually transmitted diseases and circumcision (see below Factors affecting the probability of HIV sexual transmission).
Additional information in the “Preventing HIV sexual transmission” section
Sharing syringes and other equipment for injecting use of addictive substances
Sharing syringes for intravenous administration of additive psychoactive substances involves a relatively high probability of HIV transmission. As the person that uses injection drugs draws blood, in order to check if the needle is in the vein, small (and usually not visible) blood quantities remain in the syringe barrel after the injection. If the blood residue is positive for HIV and if the same syringe is then used by another individual, it is possible for the virus to enter directly into the bloodstream. In addition, although HIV cannot survive in the external environment, it has been found that it can survive for up to 6 weeks isolated inside a syringe .
The probability of transmission remains high even if the injection is subcutaneous or intramuscular (many people that use drugs mistakenly believe they can avoid a possible infection in this way). Also, HIV transmission can occur during the sharing of other equipment used for the preparation of the substance, which may carry blood residues, such as lids, spoons and anything that can be used as a container, or cotton, fabric and anything that can be used as a filter . Finally, drug use increases the general risk of HIV transmission, as it makes the person more prone to unsafe sexual behaviors (e.g. sex without the use of condom).
For more information go to “HIV precautions when injecting addictive substances“
Rare transmission routes
Although quite rarely, HIV can be transmitted:
- through oral sex without using a condom
- from an HIV-positive mother on antiretroviral treatment to her fetus or newborn baby during pregnancy, delivery or breast-feeding
- through open wounds or sharing sharp items
- through blood transfusion or other blood products and organ transplants, if no precautions are taken
Vertical transmission (from mother to fetus or infant)
Without the use of any protective measures, the probability of HIV transmission from mother to fetus or infant during pregnancy, childbirth or breastfeeding is estimated at 25-45% . Transmission is most likely to occur during childbirth, when the baby comes into extensive contact with the mother’s blood and genital secretions. Breastfeeding also involves a relatively high transmission risk, as HIV contained in breast milk can penetrate the infant’s immature digestive system. Finally, though it is rare, transmission may occur during pregnancy, especially if placental injury occurs and the mother’s blood passes into the fetal circulation .
However, if a series of precautionary measures are taken, the probability of HIV transmission from mother to child is currently less than 1% . The most important prevention step is the timely use of antiretroviral treatment by the pregnant woman, so that viral load reduction to an undetectable level is achieved (especially during the third trimester of pregnancy). Other protective measures that are taken are cesarean delivery, prophylactic antiretroviral treatment for the infant immediately after birth, avoidance of breastfeeding and general care of the mother’s health during pregnancy.
Injuries or sharing of sharp objects
If a person gets injured by a sharp object, which bears residues of fresh blood that is positive for HIV, a very low probability of transmission exists (estimated at around 0.3% ), as it comes in direct contact with the bloodstream . The transmission probability depends on the depth and extent of the injury, the amount of blood residue and mainly the viral load it contains . The risk of injury with a sharp object is, as a matter of fact, greater, when it comes to health professionals (doctors, nurses etc.), however everyone should generally avoid sharing items used for personal hygiene that may carry blood residues, such as razor blades, shavers, scissors, toothbrushes. Similarly, when someone goes through a kind of cosmetic surgery, such as tattooing or body-piercing, he/she needs to make sure that sterilized equipment is being used. An injury from a dropped, used syringe can theoretically lead to HIV transmission, but this is extremely difficult to happen, as the wound will most likely be superficial and the blood residue non infectious. Worldwide no such incident has been recorded .
For more information go to “HIV prevention in everyday life and workplace”
Oral sex without the use of condom
The likelihood of HIV transmission during oral sex without a condom is still a subject of concern for researchers, as the available epidemiological data are insufficient in order to draw certain conclusions . It is however known that the anatomical and biological features of the mouth and esophagus are such that make a probable transmission very difficult, although not impossible. When there are no wounds or bleeding spots, the oral and esophageal mucosa is much less susceptible compared to the rectal, vaginal and penile ones, due to the low concentration of immune cells that can be infected by HIV. In addition, saliva contains an enzyme (SLPI), which to a large extent inactivates the virus . However, it is certain that, although rarely, HIV transmission actually occurs through oral intercourse without the use of condom.
The likelihood is greater for the partner that gives oral sex to his/her partner. HIV can be transferred through the sperm and pre-ejaculatory fluids (in the case of men) or through the vaginal fluids or period blood (in the case of women) from the receptive partner into the mouth of his/her partner, especially if there are wounds or bleeding spots . Ejaculation inside the mouth and swallowing of sexual secretions also increases the likelihood of transmission. In contrast, the transmission of HIV from the mouth of a partner to the genitals of another is considered extremely difficult and by many experts impossible [14,17]. There is no reliable report of HIV transmission from the mouth to the genitals.
Additional information in the “Preventing HIV sexual transmission” section
Medical operations (blood transfusion, organ transplantation)
For several years now, transfused blood, blood derivatives and organs used for transplantation are thoroughly tested through laboratory tests, for HIV and other infectious agents. In addition, when it comes to the selection of blood or organ donors, a careful process of selecting and excluding people that are thought to have an increased likelihood of being positive to HIV is followed. In countries where such process is systematically applied, including Greece, there is almost no probability of HIV transmission through medical procedures. In theory, there is still a very low possibility of HIV transmission through blood transfusion or organ transplantation, if these are examined through the HIV window period (see Testing for HIV) and the result is falsely negative. Such probability has been estimated at 1/1,467,000 by a relevant American study .
Other extremely rare cases
In the relevant literature, there is a very small number of reports regarding HIV transmission in other extremely unusual ways, such as the violent fight between a HIV-positive and HIV-negative person, both of which caused extensive injuries and blood-to-blood contact occurred [19,20]. Also, few cases of transmission have been reported in people that offered first aid to someone injured in a traffic accident, positive to HIV and with extensive bleeding . Finally, there is a very small number of reports of HIV transmission from blood drops that came in contact with the ocular mucosa .
These cases concern modes of transmission, which – in theory – are biologically possible, though can occur under many conditions. However, these reports are extremely rare and most of them old and unreliable, as the possibility of transmission in some other way cannot be excluded with certainty. In any case though, if someone thinks that he/she has been exposed to HIV, even in any of those unusual ways, the best he/she can do is to wash the wound or the part of possible exposure with plenty of water and soap (without pressure or rubbing) and directly head to a hospital or an AIDS Referral Centre, in order to assess the need for prophylactic treatment (PEP) .
For more information go to “HIV prevention in everyday life and workplace”
Improbable transmission routes
It is impossible for HIV to be transmitted by:
- non-sexual, every-day physical contact and kissing
- sharing household items
- sharing food or water
- sharing toilets
- mosquito or other insects bites
It is biologically and statistically impossible for HIV to be transmitted in any of the following ways:
Simple physical contact
There is no chance of HIV transmission, if someone touches, caresses, massages, embraces or holds hands with a HIV-positive person. HIV cannot penetrate intact skin and is not contained in infectious amounts in sweat, saliva or tears . HIV cannot be transmitted through the air or by breathing and is rapidly disintegrated, when exposed to the external environment . Worldwide there has been no single case of HIV transmission through simple (non sexual) physical contact.
It is impossible for HIV to be transferred through saliva, and therefore kissing, even the deep, prolonged one does not involve any transmission probability. The mucous membrane of the oral cavity and esophagus is far less susceptible to HIV compared to the genital and rectal ones, due to the low concentration of cells that can be affected by HIV and due to the action of the SLPI enzyme, which is contained in saliva and largely inactivates the virus . In theory, there is a minimal transmission probability, if both partners have wounds and widespread bleeding inside the mouth, though there are no such reliably documented cases.
Sharing household objects
There is absolutely no chance of HIV transmission through the sharing of household items such as dishes, glasses, cutlery, towels and clothes, chairs, tables or other kinds of furniture, paper, switches, knobs and other surfaces. Even in the rather rare case, when HIV-positive blood residues are found in the above mentioned objects, the virus decomposes very quickly, when exposed to the external environment . In addition, HIV cannot penetrate intact skin. Worldwide there has been no case of HIV transmission through the common use of household items.
Food and water
There is absolutely no chance of HIV transmission by consuming food that has been prepared by a HIV-positive person. Even in the rare case, when food gets mixed with blood, the exposure of the virus to the air, the food acids, the cooking heat and, finally, the stomach acids that are produced during digestion, eliminate it immediately. Similarly HIV cannot be transmitted in case we eat from the same plate or drink from the same glass with a HIV-positive person. It is impossible for HIV to be transferred through saliva and the stomach is not a very hospitable place for most microorganisms. The only and very few reported cases of HIV transmission through food, concern HIV-positive mothers, who gave their babies food they had previously chewed, while having open mouth sores .
There is absolutely no chance of HIV transmission by sharing a toilet, even if it comes to public restrooms (there is, however, a risk of transmission for other infectious diseases). HIV is not contained in infectious amounts in urine or feces. Even in the rare case, when there is HIV-positive blood (e.g. period blood) or semen and we come in contact with it, the virus survives for a very short period of time in the external environment . Besides, it is impossible for HIV to penetrate intact skin. Worldwide there has been no case of HIV transmission by toilet sharing.
Mosquito and other insect bites
There is absolutely no case of HIV transmission through a mosquito or any other insect bite and worldwide there has been no such incident, even in countries with large mosquito populations and high prevalence of HIV . When a mosquito bites a human, it does not inject any blood of its own or of people it had bitten before. Besides, mosquitoes do not bite different people repetitively, but need some time to “digest” the absorbed blood, before heading to the next target. During this time period, the virus is very quickly eliminated by the mosquito’s digestive enzymes. However, through a mosquito bite, a small amount of saliva is injected, which facilitates the absorption of blood and may contain other viruses (e.g. the malaria virus).
Factors that affect the probability of HIV sexual transmission
- A series of factors can increase (e.g. high viral load, presence of other STIs, use of poppers) or decrease (e.g. being on antiretroviral treatment and having an undetectable viral load, circumcision) the possibility of HIV sexual transmission.
The factor that has been found to affect the likelihood of HIV transmission more than any other is the viral load, the amount of viral genetic material (HIV RNA) that circulates inside the body. The viral load is determined by specific blood tests, which measure the number of copies of viral RNA per ml of blood (copies/ml) (see Important medical examinations). A viral load of more than 100,000 c/ml is generally considered high and indicates that HIV is rapidly multiplying, causing a parallel reduction to the number of CD4+ cells, whereas a viral load of less than 10,000 c/ml is considered as relatively low. When the viral load is lower than 50 copies per ml of blood, it is considered undetectable. This does not mean that HIV has been completely removed from the body, but that its levels in the blood are so low that it cannot be detected and measured by conventional laboratory tests.
Many studies have shown that there is a direct and clear correlation between the viral load and the likelihood of HIV transmission: the higher the viral load, the higher the probability and vice versa [24,25]. For instance, in a 2009 study it was estimated that if 1,000 HIV-positive individuals (male or female) with a viral load of less than 400 c/ml have regular vaginal intercourse with a seronegative partner, only 1 transmission is to be expected in one year. If the viral load is higher than 50,000 c/ml, at least 90 transmissions are to be expected in the same period of time . In the most recent study that examined HIV transmission through heterosexual intercourse, it was estimated that the transmission probability per contact is 1/3571 when the viral load is 1,000 c/ml, 1/1220 when the viral load is 10,000 c/ml, 1/416 when the viral load is 100,000 c/ml and 1/147 when the viral load is 1,000,000 c/ml .
The viral load reaches its highest point during the time of acute HIV infection, in particular the first 6-8 weeks after the exposure to the virus (see The stages of HIV infection). During this period, when it is mostly unlikely for someone to be aware of having contracted HIV, the transmission probability is much higher compared to any other stage of the infection (apart from the advanced stage of AIDS) . However, the fact that the acute HIV infection lasts for only a few weeks largely limits the number of transmissions at this stage.
The definite and positive correlation between viral load and transmission probability has turned the medical community’s attention to the examination of the use of antiretroviral treatment as means of preventing HIV transmission. Antiretroviral therapy, if used properly, reduces in most cases the viral load to a non detectable level within 3 to 6 months (see What is antiretroviral treatment?). Indeed, large scale studies have shown that the systematic use of antiretroviral treatment drastically reduces (up to 96%) the likelihood of HIV transmission in heterosexual couples . Whether this happens to the same extent, when it comes to men that have sex with men, is not yet clear. Preliminary data from the ongoing PARTNER study indicate a similar pattern (no case of HIV transmission in homosexual couples during condomless intercourse, when the HIV-positive partner has undetectable viral load), though more robust results are expected in 2019 .However, the above mentioned data have had a huge impact on the medical community. The World Health Organization, since 2015, recommends the immediate initiation of antiretroviral therapy to all people that live with HIV, regardless of their number of CD4+ cells, for the protection of both individual and public health  (see Starting antiretroviral medication).
Other sexually transmitted diseases
The presence of another sexually transmitted disease, apart from the HIV infection (e.g. genital herpes, syphilis, gonorrhea, chlamydia), significantly increases the risk of HIV transmission during sexual intercourse without the use of condom [32,33]. This happens for many reasons. First of all, many sexually transmitted diseases cause ulcers or wounds to the skin and genital mucosa. This means on the one hand for the seronegative partner, that HIV has more points of access to his/her body and on the other hand for the HIV-positive one, that HIV has more ways out of his/her body. In addition, the presence of inflammation to the skin or the genital mucosa attracts to the area a large number of immune cells, so in this way HIV has more potential targets. Finally, sexually transmitted diseases weaken the mucosal immunity, making it more susceptible to a possible infection.
A series of epidemiological studies have provided strong evidence that the probability of HIV transmission through vaginal sexual intercourse to men that have been circumcised is significantly (up to 60%) lower [34,35]. In this way, the World Health Organization recommends circumcision as a key measure for HIV prevention, especially when it comes to African countries . This drastic decrease in the transmission probability is due to the fact that a large number of Langerhans cells is concentrated in the plexus (the folding of the skin that covers the glans). These are the cells that are primarily infected by HIV, during condomless sexual intercourse and through which HIV is transferred to the lymph nodes . Therefore, circumcision largely reduces the possible points of access for HIV.
Poppers (alkyl nitrites) are a group of inhaled drugs, particularly widespread among men who have sex with men, especially in countries of Western Europe. Their use has been associated with anal sex, as they have the ability, among others, to cause relaxation of the rectal muscles and increase blood flow to the vessels of the area, which is thought to facilitate intercourse and make it more enjoyable. However, related studies have shown that their use doubles the likelihood of HIV transmission to the receptive partner during sexual intercourse, specifically due to the vasodilatation induced in the rectum area [38,39] and possibly by making individuals more prone to unsafe sexual practices.
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