Tuesday, August 23, 2005

Condoms and Seatbelts

There are three ways in which a large increase in condom use could fail to affect disease transmission.

First, condom promotion appeals more strongly to risk-averse individuals who contribute little to epidemic transmission. Second, increased condom use will increase the number of transmissions that result from condom failure. Third, there is a risk-compensation mechanism: increased condom use could reflect decisions of individuals to switch from inherently safer strategies of partner selection or fewer
partners to the riskier strategy of developing or maintaining higher rates of partner change plus reliance on condoms. A Canadian study cited by Wilden (Wilde GJS. Target Risk. PDE Publications, 1994.) showed that televised AIDS messages from the Ontario Ministry of Health made respondents more inclined to use condoms and less inclined to avoid casual sexual partners. A US study showed that women taught to negotiate condom use with their partners had no change in incidence of sexually transmitted disease compared with controls, with a trend to an increase in such diseases.
(Cohen DA, Dent C, MacKinnon D, Hahn G. Condoms
for men, not women: results of brief promotion
programs. Sex Transm Dis 1991; 19: 245-51)

A vigorous condom-promotion policy could increase rather than decrease unprotected sexual exposure, if it has the unintended effect of encouraging greater sexual activity (figure 3)[Not available]. The figure shows the potential effects of increasing condom use among soldiers posted overseas for 6 months, when the condom failure rate is 10%. Data are derived from the work of Hopperus-Buma and colleagues,

(Hopperus-Buma AP, Veltink RL, van Ameijden EJ, Tendeloo CH, Coutinho RA.
Sexual behaviour and sexually transmitted diseases in Dutch marines and naval personnel on a United Nations Mission in Cambodia.
Genotourin Med 1995; 71: 172-75)

and use the equation: total number of acts of unprotected sexual intercourse=total
number of all acts of sexual intercourse (1minus c)doplus(cxf) , where f is the proportion of acts in which the condom fails, and c is the proportion of acts in which a condom is used. Point A shows that if sexual intercourse takes place on a mean of two occasions per soldier, there will be 1100 acts of unprotected sex per 1000 soldiers if condom use is 50%. Point B shows a fall in unprotected sex of 33% to 740 acts, which could be achieved by increasing condom use from 50% to 70%. Point C shows that if, as a result of condom promotion and availability, the mean number of episodes of sexual intercourse per soldier increased from two to three, the benefit of increasing condom use from 50% to 70% would be lost. Point D shows that a doubling of acts of sexual intercourse (from two to four) would lead to a substantial (35%) increase in the amount of unprotected sex if condom use is increased to 70%. Point E shows that condom uptake would need to increase to at least 81% to bring the level of unprotected sex back to baseline. Points Aprime to Eprime relate to a baseline situation of 10% condom use. In this case, to reduce
the total number of unprotected sexual acts, condom use must increase to at least 44% if the total number of acts increases by 50%, and to at least 61% if the total number of acts doubles. .... These findings can be generalised.
(The Lancet; 355 (9201): 400-403
January 29, 2000
Condoms and seat belts: the parallels and the lessons
Department of Sexually Transmitted Diseases, Royal
Free and University College Medical School,
The Mortimer Market Centre, Mortimer Market,
London WC1E 6AU, UK
Richens, John; Imrie, John; Copas, Andrew)

A note on behavioral adaptation:
There is evidence to show behavioural adaptation in response to other interventions that may affect HIV transmission. Two studies have reported that gay men are less worried about HIV infection since treatments have improved, and that they are significantly more likely to report unprotected sexual exposure than in the past. Kalichmann reported that of 327 men surveyed at a Gay Pride festival in Atlanta in 1997, eight (3%) had already used antiretroviral post-exposure prophylaxis and 85 (26%) intended to do so if the occasion arose. Otten and colleagues showed that rates of prevalence of sexually transmitted diseases doubled in a group of patients who had a negative HIV test and counselling for prevention. Early studies on the likelihood of HIV transmission through oral sex suggested
that transmission by this route was insignificant, which led to widespread advocacy of oral sex as a safer alternative to anal sex for gay men. Since then
there has been a steady increase in the number of transmissions attributed to oral sex, which has led epidemiologists to revise upwards their estimates of the likelihood of transmission from oral intercourse.

In an interesting theoretical paper Blower and McLeann have argued that a suboptimal HIV vaccine might increase transmission if lowered risk perception in the target population led to increased risk behaviour.