How soon should PEP be used?

When used, PEP should be initiated promptly. Animal research suggests that PEP may be ineffective if started later than 24 to 36 hours after exposure. The animal study upon which these estimates are based comes from macaque monkeys given tenofovir as the sole PEP drug.1

In this study, 24 macaque monkeys were divided into six groups of four. They were all given a dose of Simian Immunodeficiency Virus (SIV - the equivalent to HIV in monkeys) ten times larger than the dose which would be expected to infect 50% of monkeys – a dose that should have infected all of them. They were then treated with tenofovir starting various times after infection and continued for various time periods. The results were as follows:

A: Control group (treated only with saline, not tenofovir: 100% infected: all seroconverted by week four.

B: Tenofovir started 24 hours after exposure and continued for 28 days: probably none infected (one monkey eventually showed antibodies to SIV after 32 weeks but free or cell-associated virus was not detected in any monkey).

C: Tenofovir started 48 hours after exposure, continued for 28 days: all seroconverted to SIV by week 16, though virus was only detectable in 50% of monkeys.

D: Tenofovir started 72 hours after exposure, continued for 28 days: all seroconverted by eight weeks, though virus only detectable in 50%.

E: Tenofovir started 24 hours after exposure, continued for 10 days: 50% eventually seroconverted, though virus only detectable in one (25%).

F: Tenofovir started 24 hours after exposure, continued for three days: all seroconverted by eight weeks, virus detectable in 50%.

It is this study that is the basis for the recommendation that PEP should be started within 24 hours of exposure, though it may be 50% effective up to 72 hours after exposure.

A 2006 survey by the Health Protection Agency2 of PEP provision at ten London HIV clinics found differences in the time starting PEP between healthcare workers and those exposed sexually or through injecting drug use. For people seeking PEP after non-occupational exposure, the average time elapsed between exposure and taking PEP was 23 hours, whereas for occupational exposure it was two hours.

References

  1. Tsai CC et al. Effectiveness of postinoculation (R)-9-(2-phosphonylmethoxypropyl) adenine treatment for prevention of persistent simian immunodeficiency virus SIVmne infection depends critically on timing of initiation and duration of treatment. J Virol 72: 4265-4273, 1998
  2. Delpech V PEP: the bigger picture. Presentation at 9th CHAPS Conference, Leeds, UK, March, 2006
This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.
Community Consensus Statement on Access to HIV Treatment and its Use for Prevention

Together, we can make it happen

We can end HIV soon if people have equal access to HIV drugs as treatment and as PrEP, and have free choice over whether to take them.

Launched today, the Community Consensus Statement is a basic set of principles aimed at making sure that happens.

The Community Consensus Statement is a joint initiative of AVAC, EATG, MSMGF, GNP+, HIV i-Base, the International HIV/AIDS Alliance, ITPC and NAM/aidsmap
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This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.

NAM’s information is intended to support, rather than replace, consultation with a healthcare professional. Talk to your doctor or another member of your healthcare team for advice tailored to your situation.