Originally posted by bounder
Because the only way I know of testing a vaccine in humans is to vaccinate them and then attempt to infect them.
Um, you don't need to infect them. You only need to find a large, sexually active sample group, vaccinate them, and then check for infection at the end of the trial period.
We know infection rates among the general population and among groups at risk, so if the rate of infection among the sample is significantly higher or lower than that of the general population, it'll say something about the effectiveness of the vaccine.
EDIT: Also keep in mind the control group. If you randomly distribute your human subjects into a vaccine group and a placebo group, you can test difference in infection rates between the two groups. As selection to each group would occur randomly, we expect that, should the vaccine be ineffective, rates of infection between the two groups would be nearly the same.
-- Todd/Leviathn