For those not familiar with this Haunt, a found a general description of Haunt #43 — The Star Sickness, (although sadly not the exact rule text, so if someone has the rules handy please update).
We played this haunt the other day and interpreted the rules to mean that a traitor was only killed by the serum when it was injected in them by an Alien, because the rule stating they were killed by the serum were in the Alien section of the Haunt description. Thus when a traitor was immunized by a player we assumed it had no effect. But as we played it out, this didn't seem to make sense.
From the link above, it appears the correct interpretation is that when a traitor is injected for any reason they die, and when a player is injected they are cured.
However, this didn't make sense to us either because then a traitor would have to reveal themselves as such when someone wanted to "cure" them. Or is this how it is supposed to work? The best way to root out traitors is to try and cure them?