GHENT UNIVERSITY Karoline FONCK - International Centre for ...

Several models have been developed for STI epidemiology. The essential concept in these
models is the reproductive rate (R0) of the infection. R0 is defined as the number of
secondary infections that a spreader produces in a fully susceptible population. For STI
transmission, R0 is determined by three variables. These are the average rate at which new
sex partners are acquired per unit of time (c), the average probability that infection is
transmitted from one person to a susceptible individual per sexual contact (ß), and the
average duration of infectiousness of an infected person (D). In the simplest possible case,
where a single risk group mixes homogeneously (chooses sex partners at random), the
transmission dynamics are approximated by the formula:
R0 = ßcD
What is meant exactly by average is not straightforward, since epidemiological studies reveal
much variability between and within specific populations. Many heterogeneities influence the
magnitude of β (sex acts per unit of time, type of sex act) and measures of this value via
studies must be interpreted with caution. Recorded estimates of the magnitude of β range
from a very high likelihood of transmission for infections such as gonorrhoea and syphilis and
to a low probability for infections such as HIV and chlamydia. The typical duration and
degree of infectiousness (D) is also subject to variation and duration often varies widely
between individuals. The duration of infectiousness can be influenced by interventions such
as chemotherapy. Treatment therefore reduced the value of R0.
The principal determinant of STI spread in a population is the rate at which new sexual
relationships occur in that population, rather than the number of sexual exposures per new
partnership. The more sexual partners one has, the greater the likelihood of encountering
high frequency transmitters of STI pathogens. Rates of sexual partner change vary widely
within and between societies, and are associated with a range of demographic and socioeconomic
factors. Invariably, the reported rate of change of sex partners for men exceeds
that for women. Surveys reveal much heterogeneity with most people reporting few partners
and a few reporting many. That core group of highly sexually active individuals however is a
major influence on the transmission pattern, as they will assure that R0 exceeds unity in
value (Yorke 1978, Anderson 1987, Anderson 1988).
Individuals with two or more simultaneous (i.e. concurrent) partners can play a central role in
the spread of infection, as the individual can acquire infection from one partner and pass it on
to the another without gaining any new partners. In both developed and developing
INTRODUCTION 5