Objectives: We present the first evidence-based method for estimating public health and cost impacts of partner notification (PN) that takes account of sexual partnership type.
Methods: Our algorithm uses routine clinical data, probability survey data, and transmission parameters. We propose 2 new epidemiological concepts to quantify PN impact: "[the] absolute reduction in onward transmission" and its reciprocal, "[the] number needed to treat to interrupt transmission" (i.e., the number of partners who need to be treated to interrupt 1 onward transmission). We demonstrate these concepts for 273 chlamydia cases diagnosed at a UK genitourinary medicine clinic.
Results: The number needed to treat to interrupt transmission (overall, for casual partners, and for regular partners, respectively) was 1.47, 1.11, and 2.50, respectively, for men younger than 25 years; 1.60, 0.83, and 1.25, respectively, for women younger than 25 years; 2.35, 1.39, and 2.08, respectively, for men older than 25 years; and 2.14, 0.93, and 2.08, respectively, for women older than 25 years.
Conclusions: PN that targets casual partners, rather than regular or live-in partners, prevents more secondary transmissions per partnership; it is also more resource intensive, but the public health benefit is greater.