Scientific American, 7/98, Susan Buchbinder, page 84
Avoiding Infection after HIV Exposure
Treatment may reduce the chance of contracting HIV infection after a risky encounter
Suppose you are a doctor in an emergency room and a patient tells you she was raped two
hours earlier She is afraid she may have been exposed to HIV, the virus that causes AIDS,
but has heard that there is a "morning-after pill" to prevent HIV infection.
Can you in fact do anything to block the virus from replicating and establishing
infection? Would you respond differently to a patient with an HIV positive sexual partner
whose condom had broken during sex? How would you treat a patient who occasionally got
drunk and had unprotected sex with someone of unknown HIV status?
Unfortunately, there are no easy answers. Only in the past few years have physicians had
any data that could help resolve these dilemmas, and much remains unknown. Several studies
suggest, however, that under certain circumstances, delivering anti-HIV medications after
exposure to, the virus - that is, giving postexposure prophylaxis (or PEP) - might prevent
HIV infection.
Laboratory experiments indicate that administering anti-HIV drugs either before exposure
or up to 24 hours afterward can protect some test animals from infection with HIV or a
related virus that infects monkeys. The strength of the protection depends on the type and
duration of the antiviral therapy.
Human studies also encourage hope that PEP can work. Transmission of HIV from women to
their babies is reduced if the mothers are treated with the drug zidovudine (AZT) during
pregnancy and labor and if the babies are treated immediately after birth. In this
situation, however, protection may result in part from zidovudine circulating in the
baby's blood at the time of exposure, rather than after exposure.
Further evidence comes from a study of health care workers exposed to a patient's HIV
infected blood through a needle stick or other accident that breaks the skin. The
investigation found that health personnel who were exposed to HIV but did not become
infected were more likely to have received PEP (with zidovudine) than were workers who did
become infected. This finding, however, does not amount to proof that PEP reduces
infection; methodological difficulties preclude drawing definitive conclusions.
Given the uncertain effectiveness of PEP, we physicians want to be sure that the treatment
does not pose unacceptable risks to our patients. We know the short-term side effects from
experience with HIV infected patients, but we know almost nothing about the long-term
consequences of using HIV fighting drugs in people who may not in fact harbor the virus.
Furthermore, we have only a very rough idea of how long PEP should be continued.
Despite the uncertainties, the Centers for Disease Control and Prevention have recommended
that all medical personnel who have a significant exposure to HIV infected blood or bodily
fluids (such as through a needle stick) be counseled and, depending on the situation, be
offered anti-HIV PEP The drugs now recommended for this treatment are some of the same
ones widely used to treat established HIV infection: zidovudine, lamivudine (3TC) and
possibly also one of the newer drugs called protease inhibitors. These agents are known to
be effective against HIV in combination, are widely available and have relatively few
serious side effects: Alternative drugs may be given, depending on the likelihood that the
source patient harbors resistant viruses and on other medical conditions the exposed
worker may have.
Although the popular term "morning-after pill" might suggest an easy fix, the
recommended prophylactic course involves taking the drugs at least twice a day for a total
of four weeks. Patients who choose PEP must be ready for a number of temporary side
effects, including headache, nausea, fatigue and anemia. They must also be willing to face
both unknown long-term risks and the fact that HIV often becomes resistant to the drugs it
encounters. If a patient becomes infected in spite of PEP, any resulting viral drug
resistance would reduce the options available for treating the infection.
Should the recommendation made for accidentally exposed health workers also be made for
people who are exposed to HIV through sex or through using contaminated needles to inject
drugs? The unsatisfactory answer is: perhaps.
Some types of nonoccupational exposure can be just as dangerous as a needle-stick injury.
The risk of becoming infected from a single instance of unprotected receptive anal sex
with a man of unknown HIV status, for example, may be as high as that from a stick by a
needle that has been used in a patient known to carry HIV The one-time risk from using
contaminated injection equipment appears to be comparable.
Yet the circumstances surrounding nonoccupational exposures differ in important ways from
those that occur in health care settings. These circumstances may lessen the effectiveness
of PEP or even increase the dangers.
Beyond Health Care Workers
Health care workers sustaining a needle-stick injury usually recognize their exposure
immediately and can obtain PEP medications without having to pay for them. Moreover, their
physicians will have access to the source patient's medical record, which means they can
tailor treatment so it is most likely to be effective. For example, if a drug had
previously been found to be ineffective at reducing virus levels in the source patient,
then the exposed person would probably need to avoid taking it. In contrast, individuals
exposed through sexual contact or drug use may not know the HIV status of a partner, and
the partner's medical record is often unavailable. Even when people immediately recognize
a nonoccupational exposure to HIV, they may find it hard to arrange quickly for medical
care and for payment to cover expensive drugs.
This barrier is a major problem, because the available evidence suggests that PEP must
begin quickly to be effective. Indeed, treatment that starts within hours of exposure is
likely to be more effective than treatment that starts a day or more later, and there is
probably little to be gained from initiating PEP more than 72 hours after exposure.
Even if a nonoccupational exposure to HIV is unequivocal and the patient seeks help
quickly, other factors may argue against treatment. Notably, if the patient cannot take
the prescribed medications as directed or is exposed repeatedly to HIV during the period
of PEP treatment, medication-resistant HIV may emerge (thus complicating future
treatment), Yet many people find it difficult to adhere strictly to therapy, and some find
it hard to practice safe sexual or intravenous drug-taking behavior consistently.
Other risks of PEP are less obvious. Merely knowing that it is available could lead some
people to take more chances than they would otherwise. Thus, there is a real possibility
that offering PEP for a nonoccupational exposure might in some cases increase the
likelihood of infection. This consideration presumably does not apply to health care
workers, who seem unlikely to become more careless just because PEP is available.
Several studies are in progress or being planned to clarify the benefits and drawbacks of
PEP for people exposed nonoccupationally, so that health care providers can make sound
recommendations. In the meantime, physicians have published some general guidelines.
Doctors confronted with a worried patient should assess the probability that HIV infection
will occur, considering the type of exposure and the likelihood of infection in the
partner Counseling should be offered and referrals made as appropriate. Because HIV
transmission during sexual assault has been reported, PEP is often recommended in such
cases. Other isolated high-risk sexual exposures (such as condom breakages) may justify
PEP, particularly for the receptive partner (for whom the risk of acquiring HIV is
highest). Because multiple courses of therapy also multiply the risks of treatment, most
physicians would not recommend PEP for persons who are exposed to HIV repeatedly.
PEP is at best a strategy of last resort in preventing HIV infection. The real hope for
halting the worldwide spread of this deadly virus is still a comprehensive preexposure
prevention program and - ultimately - an effective vaccine.