Portfolio theory, part II

With most infectious diseases, reducing everyone’s risk by a third would make
quite a difference across a whole population. But the problem with HIV is
that it is both an infectious disease and a behavioural one. I can get it by
sharing needles with other drug injectors, I can avoid it by using condoms
every time I have sex. If I know I have been vaccinated, will that make me
more likely to share needles, or less likely to use condoms? And if it does,
will that change outweigh the 30 per cent reduction in risk that comes with
the vaccine?

That is Elizabeth Pisani, here is more.