  Sexual contacts are the main spreading route of HIV. This puts sex workers at
higher risk of infection even in populations where HIV prevalence is moderate
or low. Alongside condom use, Pre-Exposure Prophylaxis (PrEP) is an effective
tool for sex workers to reduce their risk of HIV acquisition. However, PrEP
provides no direct protection against sexually transmitted infections (STIs)
other than HIV, unlike condoms. We use an empirical network of sexual contacts
among female sex workers (FSWs) and clients to simulate the spread of HIV and
gonorrhea. We then investigate the effect of PrEP adoption and adherence, on
both HIV and gonorrhea prevalence. We also study the effect of a potential
increase in condomless acts due to lowered risk perception with respect of the
no-PrEP scenario (risk compensation). We find that when HIV is the only disease
circulating, PrEP is effective in reducing HIV prevalence, even with high risk
compensation. Instead, the complex interplay between the two diseases shows
that different levels of risk compensation require different intervention
strategies. Finally, we find that providing PrEP only to the most active FSWs
is less effective than uniform PrEP adoption. Our work shows that the effects
emerging from the complex interactions between these diseases and the available
prophylactic measures need to be accounted for, to devise effective
intervention strategies.
