You can randomize it in an authoritarian way if you reject here the free market, the way of selection matter far less than the discussion of how much lives could be saved.

What a repugnant stance you've adopted.
Thinking like you think is consenting to mass murder, do you realize it ?

Here's my calculus: The phase I trial has approximately lasted 2 months.
I checked the number of total deaths from 01 may to 01 July -> https://www.worldometers.info/coronavirus/worldwide-graphs/#...

This give us 277,145 human deaths in this trial period.
According to https://en.wikipedia.org/wiki/Mortality_due_to_COVID-19#Mort... mortality rate for a ~20 years old should be at least lower than 0.1%.

With all those data in mind, here's the reasoning:
The metric of effectiveness for this Moderna vaccine candidate is epistemologically LOW. They use fuzzy heuristics of matching approximately the immune response of naturally recovering patients as a claim of being effective. So firstly this reasoning is simplistic especially since researchers do not understand precisely how the working immune response work.
So there is a risk of the vaccine to be a disappointment. If it is, those 277,145 human lifes would have been wasted and far more will die until we find the true vaccin. But what you need to understand is that researchers are condemned to match the existing immune response as otherwise, without testing on infected young humans, they cannot predict the vaccine expected performance. This alone is far enough for defending that they should have tested on a few dozen of infected patients the effectiveness.
If they did: two possible scenarios:
(lets suppose they tested 30 humans in their twenties that would make 0.3% of chance of deaths (comparatively to 277,145 human deaths) and the likelihood would be actually far lower as they would be actively monitored)
scenario A) the vaccine doesn't work and it shows empirically, it save them weeks of evidence and therefore make them reconsider another paradigm for the virus. That alone would save at the very least thousands of lives by allowing the true vaccine to be found earlier. So you must realize that your "repugnant stance" of inaction is responsible in this likely scenario of far more deaths, sadly numbers quickly become unproportional to the emotional empathic response for a homo sapiens, this kind of brain failure honestly give a taste of vomit in my mouth, but yeah everybody is victim of this cognitive bias, even myself so no offense.
In the second scenario, the empiricism would show them that their vaccine works: here again it would accelerate the delivery, and crucially the data would give them insights and allow for far better fine tuning of how the vaccine works and the optimal dosage.
Is is not unreasonable to think that such speedup would allow to save a similar order of magnitude than 277,145 human deaths but could actually be far more (imagine 6 months of work saved y avoiding empirically invalidated or fine tuned research directions ?
Even if it were to be lower, even if the data sped up the research by only one day (an insanely pessimistic view that I only show to defend your reasoning to the extreme, to its limit) would still save more than 5000 lives which is indeed far bigger than the likely lower than 0.3% of death rate for ~30 persons, so even in the ridiculous limit we talk about 5000+ vs 0.3 death.