15th International AIDS Conference Bangkok, Thailand - July 11-16, 2004

Int Conf AIDS 2004 Jul 11-16; 15:(abstract no. ThOrC1430)

Vickerman PT, Foss AM, Watts CH, Rowley J
London School of Hygiene & Tropical Medicine, London, United Kingdom

BACKGROUND: Concern has been raised that after vaccine introduction people may increase their sexual risk behaviour. This study uses a mathematical model to identify the boundary threshold where the protective effect of a vaccine is counter-balanced by increases in risk due to changes in sexual risk behaviour following vaccine introduction.

METHODS: The probability that a susceptible person is infected per month before the introduction of a preventive vaccine was compared with that afterwards. The possibility of increased risk behaviour following vaccination, and any associated increases in the underlying levels of other STIs were incorporated. Two types of preventive vaccines were considered, the first reduces the susceptibility of uninfected individuals (type 1) and the second reduces the infectiousness of infected individuals (type 2). The take t (percentage of the population receiving the vaccine that show a protective immune response) and degree d (percentage reduction in the per act probability of infection among individuals with an immunological response) were varied.

RESULTS: Figure 1 shows the boundary thresholds for sexual behaviour change for a type 1 vaccine with different levels of efficacy, introduced into a low to medium risk population, where the initial consistency of condom use is 50%.<IMG SRC="images/prog/ThOrC1430_IMG02.jpg" border=0>The figure illustrates that, where 50% condom consistency was being achieved prior to vaccine introduction, if the efficacy (td) of a vaccine is 40% then sexual risk must not increase by more than 70% for the vaccine to have a beneficial effect. However, if the efficacy of the vaccine is 80% then sexual risk can increase by more than 300% and the vaccine will still have a beneficial effect.

CONCLUSIONS: The level of behaviour change that can occur without negating vaccine impact is dependent upon the setting in which the vaccine is being used and vaccine efficacy. The introduction of lower efficacy vaccines need to be accompanied by strategies to warn against increasing risk behaviour.

040711
ThOrC1430

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