This is not a current situation, but reflects a real set of circumstances that does actually happen.
Assuming (as the linked question does) that getting a shot poses a problem on Shabbat, can one get a shot that prepares one to save a life after Shabbat? Note -- not to have one's life saved after Shabbat but to participate in saving someone else's life after Shabbat.
Is the shot (in addition to any other problem) hachana for after Shabbat or is hachana suspended if the preparation is to save a life? What if the saving is not 100% inevitable?
Are there other questions one would have to ask to be able to decide the permissibility of getting the shot?