Post-exposure prophylaxis after HIV risk contact


In comparison with other viruses, such as hepatitis viruses, this pathogen actually has a relatively low risk of infection. Not every contact with the virus (exposure) also leads to an infection. Since HIV infection is still not curable, one should nevertheless avoid any risk and inform oneself in detail about all preventive measures.

With post-exposure prophylaxis (PEP), an infection can often still be averted.

The faster it occurs, the greater the probability of a successful prevention of HIV infection.

If contact with HIV has nevertheless occurred or is suspected, the so-called post-exposure prophylaxis (PEP) is a drug measure that can often still avert an HIV infection. The faster this prophylaxis takes place – preferably within 24 to 48 hours after contact with the virus – the greater the probability of a successful prevention of HIV infection. First, however, it must be clarified whether an infection is probable or possible at all.

How is HIV transmitted?

HIV is transmitted via body fluids. Blood and sperm are much more infectious than vaginal fluid or breast milk. Saliva, sweat, tear fluid, urine and stool also contain viruses, but not enough to cause an infection.

HIV risk situations include:

  1. Unprotected vaginal, anal or oral intercourse with an HIV-infected person.
  2. Unprotected vaginal, anal or oral intercourse with a person of whom HIV status is unknown.
  3. the use of HIV-contaminated hypodermic needles in substance use
  4. transfusion of HIV-infected blood or infected blood products
  5. an injury with HIV-contaminated instruments (e.g. needles, surgical instruments)
  6. Contact of mucous membrane or open wounds with an HIV-contaminated fluid

What is the respective risk of HIV infection?

The risk of infection strongly depends on the route of transmission, the type of body fluid transmitted, the viral load (amount of virus) of this fluid and the duration of exposure. For example, the joint use of HIV-contaminated needles (“needle sharing”) poses a particularly high risk of infection because HIV-positive blood is introduced directly into the bloodstream. In addition, the longer an infectious body fluid remains in contact with mucous membranes or injured skin, the greater the risk.

Risk in the occupational context

Studies from the hospital sector provide concrete figures on the probability of infection. In hospitals and laboratories, accidents with HIV-positive blood occur again and again, which is why PEP is also used comparatively frequently there. The results of scientific studies show that the risk of infection is rather low: if a skin injury is caused by an infected object, the average risk of HIV transmission is 0.3 percent. This means that, on average, one in 330 such exposures leads to HIV infection. In general, the risk of deep stab wounds or cuts is much greater than that of superficial skin wounds. If HIV-infected blood reaches a mucous membrane (e.g. the eye), the probability of infection is only 0.03 percent.

Risk through sexual intercourse

While HIV-PEP was initially only recommended in the medical profession, it has now been offered for several years after sexual risk contacts. The probability of HIV transmission through sexual intercourse depends on the type of intercourse, whether one is “active” or “passive”, whether minor injuries occur and whether a sexually transmitted disease is present.

Anal intercourse entails the highest risk of infection. It is estimated that this type of sexual intercourse with an HIV-positive partner has a transmission probability of 0.82 percent per act. Penetrating vaginal intercourse is associated with an estimated infection probability of 0.03 to 5.6 percent. 

The risk for the receiving (“passive”) person is usually higher than for the invading (“active”) person. In oral sex, the active partner who stimulates the intimate area of the other person with his mouth usually bears the greater risk – even if HIV infections caused by oral sex are only known in isolated cases. Vaginal secretions, in particular, are usually too low to lead to HIV infection if the oral mucosa is intact.

It should be noted that these figures are purely statistical. In practice, a single risk contact can lead to an infection. 

Factors that influence the risk of infection:

Stage of HIV infection

If the HIV infection is acute – i.e. at a very early stage – there is a very high virus concentration.  Acute HIV infection can temporarily trigger physical symptoms that are not specific to HIV. This means that the symptoms can also indicate other viral infections, such as a cold, the flu or Pfeiffer’s glandular fever. The same applies to HIV-infected people in later stages of the disease who are unaware of their infection or do not have it treated, as well as patients who have already developed AIDS.

Antiretroviral therapy

If the HIV-positive person who could have been infected receives an antiretroviral therapy that has lowered the virus concentration below the detection limit, the risk of transmission is practically non-existent under certain conditions. PEP, for example, is not recommended for unprotected sexual contact with an HIV-infected person without a detectable viral load because the risk of infection is so low. In this context we speak of U=U (below the detection limit = non-transmissible): If the virus is below the detection limit, the person is practically not infectious.

Primary care

In the case of stab wounds and cuts, immediate disinfection of the wound reduces the risk of infection. If the eye or oral cavity has come into contact with an HIV-contaminated fluid, it should be rinsed immediately to reduce the risk of infection.

After exposure to sexual intercourse, the penis should be washed with soap under running water. Vaginal or intestinal flushing after sexual intercourse is not recommended.

Immune system

Last but not least, the immune defence of the person who has come into contact with HIV also plays a role. Existing diseases, such as cancer, reduce the immune system. In addition, sexually transmitted infections such as herpes simplex or human papillomaviruses (HPV) increase the risk of infection during unprotected intercourse with an HIV-infected person.

Introduction of post-exposure prophylaxis (PEP)

Since doctors at HIV centres have the most experience with PEP, it is advisable to visit such a centre. Whether PEP is then actually initiated is decided after a consultation and an assessment of the risk of infection. The aim is to weigh the benefits against the risks of post-exposure prophylaxis. Therapy with HIV drugs is also associated with side effects.

Start PEP quickly

For a PEP to be successful, it must take place as soon as possible after contact with the HI virus. A first administration within two hours after the risk contact is optimal, although in many cases not realistic. In general, the best results can be expected when starting within 24 hours.

If more than 72 hours, i.e. three days, have passed since exposure, a PEP is no longer useful in most cases. In this case, HIV antibody tests can be carried out at intervals of, for example, six and twelve weeks after the risk contact in order to be able to initiate therapy as early as possible in the event of an actual infection.

HIV rapid test

In some cases it is not known whether the person with whom the risk contact has taken place is actually HIV positive.  If this person is still in contact and the person agrees, a rapid HIV test can be carried out in this situation. A test result is thus available within a few hours, whereby the diagnostic window must also be observed here.  

Whether PEP is associated with treatment costs depends on the individual case.  In the case of occupational exposure, the insurance company bears the costs. An anonymous execution of the PEP is not possible.

Effects and side effects of PEP

PEP consists of a combination of three drugs that are also used in the antiretroviral treatment of HIV/AIDS. Animal studies suggest that the mechanism of action of PEP may be based on the blockage of the initial infection and the stimulation of the immune system.

Course of PEP

The medication must be taken for four weeks without interruption. If you have started PEP relatively late, the prolongation of the medication may make up for this.

Several HIV tests are carried out during PEP in order to be able to diagnose any infection quickly. Whether the prophylaxis was successful, however, is only known six weeks after the end of therapy.

Side effects

All drugs used in HIV therapy intervene massively in the organism and therefore often lead to side effects.  People treated with PEP often complain of gastrointestinal complaints, headaches and fatigue. In pregnancy and lactation as well as in liver and kidney problems, the advantages and disadvantages of this measure must be weighed critically.

How is HIV infection diagnosed?

The most important diagnostic tool for detecting an infection with the virus is the HIV test. In this procedure, antibodies against the HIV virus are searched for in the blood.

The diagnostic window

It takes two to six weeks after an infection until so many antibodies have formed that they can also be detected in a laboratory test. At the latest six weeks after a possible infection, a modern HIV antibody/antigen test, as it is used in the western world, is meaningful. This period is called the diagnostic window.

This time span must be taken into account when interpreting the results: A negative test result – “negative” in this case means that no antibodies could be detected – therefore only excludes an infection if the last situation associated with an infection risk was at least six weeks ago.

Detection of antibodies

In diagnostics, a search test for HIV antibodies is always carried out first. If this is “positive” – i.e. antibodies against the HIV virus have been detected – the result must be confirmed by another test. This confirmation test now uses a different procedure, the so-called Western blot. If the result is positive, HIV infection is almost certain.

Detection of the virus

In addition to the search for antibodies against the pathogen, it is also possible to detect the virus itself or one of its components. However, this method is primarily used to monitor the course of the infection and the success of treatment.

Modern addiction tests combine both test principles and detect virus components (antigens) as well as antibodies.


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