Post-exposure
prophylaxis (PEP)
has been in use, initially primarily for healthcare workers, since 1988.1
Antiretroviral
treatment [ix]at this stage is believed to block the infection of immune system
cells by HIV, so prompt treatment is likely to block the establishment of HIV
infection in an individual who has been exposed to the virus.
Healthcare
workers who may have been accidentally exposed to HIV-infected body fluids (for
example through injury with a contaminated syringe) are offered a one-month
course of antiretroviral drugs. Increasingly, people who are exposed to HIV
outside the workplace, through sex and needle-sharing, are also seeking out and
obtaining PEP.
PEP has
never been subject to a randomised controlled trial as it would be unethical to
offer placebo treatment, or none, to someone exposed to HIV. In addition, over
a limited period of time, events that are known to involve a maximal exposure
to HIV, and transmission during those events, are comparatively rare, even in a
high-risk population.
For
these reasons it is hard to say what would have happened if people had not
taken PEP. A very large trial would be needed to demonstrate the effectiveness
of PEP. However a 1997 case control study2
of healthcare workers from France, Italy, the UK, and the US came to the
conclusion that PEP – in this case AZT (zidovudine, Retrovir)
monotherapy – reduced the risk of becoming infected with HIV by 81% (with a
confidence interval of 48-94%). This was equivalent to reducing the risk of
infection from 1 in 200 to 1 in 10,000.
Providing
PEP after exposure through sex (PEPSE) or, to use a more general term, which
includes parenteral exposure, ‘non-occupational PEP’ (NONOPEP), has taken
longer to become accepted and to be provided. Resistance to the idea of the
widespread provision of PEP has been based on arguments about its expense and
cost-effectiveness, whether its provision would cause increases in risk
behaviour, and lack of effectiveness on a population level.
In fact PEP
tends to be used comparatively rarely. This may be because obtaining PEP is
inevitably time-consuming and often difficult, because people often do not know
they have been at risk or believe they have not been, and because there is at
most a 72-hour period after exposure to HIV during which PEP is recommended.
There is
little evidence that the provision of PEP contributes significantly to risk
behaviour.
Although,
for these reasons, PEP is unlikely ever to make a significant contribution to
the reduction of HIV incidence and prevalence, it is clearly efficacious on the
individual level and there is a strong human rights argument for its public
provision, especially when it is already provided for occupational exposure.
A
comparison of per-exposure risks suggests that unprotected receptive anal sex3
with an HIV-positive partner may be ten times as risky, in terms of the
likelihood of infection from a single exposure, as a needlestick injury.4
And although a single needlestick injury may be a little bit riskier than a
single act of receptive vaginal sex, factors such as high viral load or trauma
during rape may make transmission more likely.5,6
One necessary
precondition for a policy of offering PEP is the widespread availability of HIV
testing, so that people considering taking PEP are sure that they are
HIV-negative. This would be especially important if the treatment option
provided were inappropriate for treating HIV-positive people, for example,
short-course Combivir, a
fixed-dose combination pill containing AZT and 3TC). It is also
important, if at all possible, to ascertain the HIV status of the contact
partner so that if they are found to be HIV-negative, PEP can be stopped.
When it
is provided, research has shown that people use PEP inconsistently. Studies
have found that many people who have sought and taken PEP have, within a few
months or even while they are taking PEP, gone on to risk further exposures to
HIV without seeking further PEP (see Putting
PEP into practice). This is not an argument
against the initial provision of PEP, but shows that people may be reluctant to
admit further risk behaviour, may leave it too late to seek PEP on subsequent
occasions, and are often bad at assessing the degree of risk they have been
exposed to; they may form very different estimates of the riskiness of an
incident even where external circumstances are the same.
Other
factors that influence the decision to seek PEP may include:
- whether the contact partner
is known to be HIV positive,
- whether the contact partner
is judged to be a ‘high-risk’ person if their serostatus is unknown,
- whether the contact partner
recommended PEP,
- whether the exposure
was consensual or non-consensual or involved trauma,
- whether it occurred
while the person was under the influence of drink or drugs,
- whether it occurred
within a casual encounter or within a primary relationship,
- psychological
factors such as depression or fatalism in the person at risk,
- and of course the
knowledge that PEP exists.