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P444 A case series of pre-exposure prophylaxis failures in men using event-based-dosing in london, UK
  1. Naomi Fitzgerald1,
  2. Achyuta Nori2
  1. 1Guy’s and St Thomas’ NHS Trust, Burrell Street Sexual Health Clinic, London, UK
  2. 2Guys’ and St Thomas’s NHS Foundation Trust, London, UK


Background British guidelines recommend both event-based dosing (EBD) and daily dosing of combined tenofovir/emtricitabine (TDF/FTC) for HIV pre-exposure prophylaxis (PrEP) for men who have sex with men (MSM). Access to PrEP in the UK is either through a clinical study with limited capacity or by purchasing online. Dosing schedules are usually self-selected, often without access to professional advice or evidenced-based information. Non-daily dosing is popular as it is believed to be less toxic and more affordable than daily dosing.

Methods We describe three cases of HIV acquisition despite use of PrEP.

Results All were MSM and had a negative antigen/antibody HIV tests at 3 months of use. Case 1 switched to EBD after one month of daily PrEP. After several months he switched back to daily PrEP and had a positive HIV-1 antibody test six weeks later. Case 2 switched to EBD after 5 months of daily PrEP due to lower frequency of sex. He reported excellent adherence. He had a positive HIV-1 antibody test 4 months later. Case 3 was taking EBD PrEP for a total of 9 months. He had a positive HIV-1 antibody 5 months after his last negative HIV test. This was two months after his last episode of condomless sex. He reported occasional late dosing and some use of recreational drugs.

Conclusion It is likely that all three cases became infected while taking event-based PrEP. Two cases switched between daily and EBD due to lower self-perceived risk of HIV or frequency of sex. It is not clear if this may have affected PrEP efficacy. Evidence for effective EBD is strongly dependent on adherence and timing of doses. It is therefore vital that PrEP provision includes objective HIV risk assessment, adherence support and evidence-based dosing information to optimise efficacy.

Disclosure No significant relationships.

  • ART
  • PrEP

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