Apples and oranges? Interpreting success in HIV prevention trials

In the last decade, several large-scale, clinical trials evaluating the efficacy of novel HIV prevention products have been completed, and eight are currently underway or about to be reported. Little attention has been given in the literature to the level of protection sufficient to warrant introduction, and there is concern that using the term "efficacy" to describe the effect of user-controlled methods such as microbicides may mislead policymakers.

We review how the fields of family planning, vaccine science and mathematical modelling understand and use the terms efficacy and effectiveness, and explore with simple mathematical models how trial results of user-controlled products relate to common understandings of these terms.

Each field brings different assumptions, a different evidence base and different expectations to interpretations of efficacy and effectiveness - a reality that could cloud informed assessment of emerging data.

When making judgments on the utility of new health technologies, it is important to use standards that yield appropriate comparisons for the innovation and that take into account the local epidemic and available alternatives.