More reasons for low heterosexual transmission in the Padian study 30 March 2003
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Peter Flegg,
Consultant Physician
Blackpool Victoria Hospital, UK FY3 8NR

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Re: More reasons for low heterosexual transmission in the Padian study

Tony Floyd (26th March) lists several reasons why Padian's heterosexual transmission study showed a low transmission rate.

There are two further important reasons. Padian recruited HIV discordant couples for the prospective part of her study.

Firstly, this means that the participants were self-selected as NOT having already acquired HIV infection from their previously infected partners. We know there are some index cases who are "poor transmitters" and similarly some people are more resistant to HIV-acquisition than others. Since Padian's study subjects had all indulged in the risky activity in question (sex) for considerable periods of time prior to joining the study, there is very likely to be a bias towards couples who were unlikely to pass on/catch HIV in any circumstance.

Secondly, we also know that HIV is maximally infectious during the acute phase of infection, and that many if not most transmissions occur during this period. All of Padian's study subjects had passed through this time frame (without experiencing any HIV transmission) before entering her study.

If an HIV seronegative individual is going to acquire HIV heterosexually, it is far more likely to happen within the few weeks after their partner has caught HIV themselves.

It stands to reason that Padian's study subjects were at substantially lower risk of HIV transmission than would be expected in a "real-life" scenario.

Competing interests:   None declared