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Does HIV pre-exposure prophylaxis use lead to a higher incidence of sexually transmitted infections? A case-crossover study of men who have sex with men in Los Angeles, California
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  • Published on:
    Error in the calculation of person-time in the before-PrEP period by Beymer et al.
    • S.H. Hulstein, Mr Department of Infectious Diseases, GGD Amsterdam

    Error in the calculation of person-time in the before-PrEP period by Beymer et al.

    S.H. Hulstein, E. Hoornenborg,  M.F. Schim van der Loeff

    Department of Infectious Diseases, GGD Amsterdam

    Studies on STI incidence and PrEP use are often hampered by the absence of STI incidence data in the period before PrEP; Beymer et al.1  set out to improve on this. They report on the STI incidence before and after initiation of PrEP in a cohort of men who have sex with men (MSM) at the Los Angeles LGBT Center, California, US. We fear that there are some flaws in the analysis, which may affect the conclusions.

    The analysis was based on 275 men who were tested at least once in the period before PrEP was started, and at least once after PrEP was started. The reported persontime in the before- PrEP period was just over half the person-time after PrEP initiation (93.60 versus 168.93), but the numbers of tests before and after PrEP initiation were not very different: 755 and 908, respectively. This discrepancy could not be explained by differences in their frequency of STI testing, which were reported to be similar in the before- and after-PrEP period. An explanation  is that the person-time before the first STI visit was not taken into account. This would mean that the person-time in the before-PrEP period was underestimated, in turn leading to an artificially high before-PrEP STI incidence....

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    Conflict of Interest:
    None declared.