Gender a factor in vaccine trialLiving with AIDS #151
South Africa ushered in a milestone event in the history of HIV/AIDS this week when the first of two human clinical trials of an HIV vaccine started in Durban and Johannesburg. Recent studies have shown that the efficacy of a vaccine can vary between men and women and so there has been a special effort to enrol equal numbers of both sexes for the trials.
Duration 4 min 23 secs
Dr TIM TUCKER: Well, it’s very important that we develop a vaccine that is equally efficacious in men and women. We know that young women acquire HIV at an earlier age, on average, than men. We know that women are often disempowered, and particularly so, within the environment of sexual empowerment. And so, it’s very important that in our trials we recruit equal numbers of men and women, so that we can have clinical trials where the findings are equally applicable to men and women.
KB: Dr Tim Tucker, Director of SAAVI, the South African AIDS Vaccine Initiative. Dr Samuel Kalibala is the East and Southern African Representative of the International AIDS Vaccine Initiative. To illustrate the importance of gender balance in recruitment for HIV vaccines, he told of two recently completed vaccine trials that suggest greater efficacy among women than men.
Dr SAMUEL KALIBALA: One is a study to test a vaccine against Herpes Simplex Virus. This study found that the vaccine’¦ had 75 % protectiveness for women, but no protectiveness for men. Now, that conclusion cannot be confirmed until we have enough samples of women. Unfortunately in that study there were few women. Another study where again there was some suggestion of difference in protectiveness is the Vax-Gen trial of HIV vaccine Phase 3 trial where 5 500 people were recruited, but only 309 of them were women. But they found that 2.7 % of the men got infected, but only 0.8 % of the women got infected.
This suggests again that the vaccine might protect women more than it protects men. But again, it is a suggestion, and can only be confirmed when we have a large enough sample of women.’
KB: The results of the Vax-Gen HIV vaccine trial were released a few months ago. The study was conducted in the US, Canada and Europe among women using intravenous drugs and men who have sex with men. The overall result of this trial showed that the vaccine did not work, but it is unclear whether making a sub-group analysis on the finding of the study is in any way helpful.
Meanwhile, Glaxo-Smith-Kline, which is testing the Herpes Simplex Virus-2 vaccine, is to commission a follow-up study using 6 000 female volunteers to validate the finding that the vaccine is more effective in women.
It is important to note that the results from these two trials are only suggestive and not facts. However, they do provide researchers with hypotheses from which to work. The question facing scientists is whether these vaccines have unique properties, whereas other vaccines, such as for measles or polio, do not show any gender-based difference in efficacy. Dr Kalibala.
Dr SAMUEL KALIBALA: We think that for these two vaccines, what is special about them, is the fact that they protect against a disease that is sexually transmitted. And there, clearly, there is a biological difference. There is a difference between the rate at which a woman gets infected from a man. So, the woman is more likely to be infected by a man than a man to be infected by a woman’¦ And therefore, maybe, that’s why the protectiveness is higher on the woman because we are working against a situation whereby she was almost a sure case of being transmitted to. Now, if anything works it will have a bigger difference for such a person than the other person who had a low chance of being transmitted to ‘ that’s the man.’
KHOPOTSO: Dr Tucker says this has implications for the vaccine trials in South Africa.
Dr TIM TUCKER: What’s important in this setting is to get adequate advice about how to design your trials. A poorly designed trial will not allow you to make sub-group analysis or analyse groups in a way that can assist us in understanding gender differences. So, all I can say to you at this stage is that we are involving high-quality scientists, and statisticians, and trial designers to ensure that down the line these hypotheses can be turned into facts that we can rely on when we eventually roll out an HIV vaccine.
KB: The trials will be conducted at two sites – the Perinatal HIV Research Unit at Chris Hani Baragwanath Hospital in Soweto, and by the South African HIV Research Unit at the Medical Research Council, in Durban.
E-mail Khopotso Bodibe