Since earlier comments indicate that humans see this as a bad bet, I can only assume that 1 in 100 people on Earth are already, in fact, aliens.

The great thing about a vaccine, even if imperfect, is it also reduces the chance that your partner is infected (if the vaccine is widely deployed). Picture the graph of how a sexually transmitted disease potentially spreads, where each node is a person and each edge is a sexual encounter. Now make 30% of the nodes immune to the disease. That's enough to protect a large chunk of graph (much more than just the 30% who are immune).