Added value of this study
infections
INFECTIONS
Our methodology uses a phylogenetic approach to identify likely transmission pairs and the direction of transmission between their members. We find a total of 300 pairs, larger than any previous study. We demonstrate a novel and simple new approach to accounting for potential sampling bias. We also employ a methodology that allows us to estimate the ages of the individuals involved at the time of transmission, rather than that of sampling, countering a key bias in previous approaches. Partly for this reason, our age profiles for sources peak at earlier ages than in previous work, with an average in the early 30s for male sources and mid-20s for females. We fail to confirm the existence of a “renewal cycle” of transmission involving a major contribution from women in older age groups. We also examine the contribution of outside-community transmission and drug resistance mutations, and, for the first time, show that these three characteristics (age and sex, migration, and drug resistance) operate on separate axes and do not cluster together. We use our results to calculate the relative contribution of male to female sources to transmission in age bands, finding that this grows to a peak in the 35–39 years age group in which men are responsible for almost six times as many new infections per person as women.