No vaccine is 100% effective and it is unethical to purposefully expose people to disease.
https://www.nytimes.com/interactive/2021/08/10/us/covid-brea...
Minor live infections to build immunity is called variolation, it's a tactic that was used effectively historically against smallpox, but effectively got obsoleted by vaccination, which doesn't kill as many people.
Despite that, let's concede the vaccine does kill people: I'm not sure that's unethical though if more people end up dead because the population didn't quickly achieve immunity and stop the spread of the virus. There are a lot of ethical frameworks out there and in the US we are actually a majority rule so as a society we should generally be protecting the interests of the majority of people except in cases where it leads to sever oppression or breach of civil liberties. We are okay with the draft (forced risk of death) during times of war yet somehow when the enemy is unfamiliar in the form of a virus we aren't?
To confirm efficacy they gave the vaccine to half of a population likely to develop the disease naturally. Then they compared how many people for sick in each group. That's why testing took a year instead of 2 months, they have to wait for enrollees to get sick naturally.
If we ignore ethics, I guess we could get more nuanced efficacy information out of a small sacrificial population. But that is just not done. Not even during a pandemic.