How is that ethical?
In one vial you have something you know to be a potentially life-saving drug.
In the other vial you have something you know to be completely useless.
How can you ethically withhold the first vial from half the residents in a nursing home where the disease is already spreading?
You're already withholding it from everyone else. Why withhold it in this case? Because there's more good to be done from determining it's efficacy than the potential saving of those few people who get the placebo in that case.
The ethical considerations seem pretty similar to any drug trail that is placebo controlled, though the stakes vary from drug to drug.