I think the parent is asking (if 1 shot of this vaccine confers 80% immunity) would it not be better (for the purposes of curbing transmission) to give everyone a single injection now for 80% immunity among a broader population rather than giving half the number of people two doses for 90% immunity. The implicit constraint is that it will take a relatively long time to scale up production and distribution to the extent that we can get two doses to everyone.

I understand what OP meant, but the data we currently have shows that one shot of this vaccine is not likely to convey a decent amount of protection.

I am sure I misunderstood something, but what does that 80% mean, and how does it differ from "the data we currently have shows that one shot of this vaccine is not likely to convey a decent amount of protection."?

The 80% number was made up in this conversation. The vaccine has not been tested using only one shot and we don't know how effective it might be.

Oops my bad

Yep, maybe they had some data about protection level rising since first shot, fewer people getting sick already etc. It's hard to understand from just the Figure 4 how much of that increase is booster shot and how much is just level rising with time, too bad they didn't investigate this. Having top level protection vaccine with 2 shots is of course great and may be what you would be aiming for in "normal" times, but when it's race against the time trading several efficiency percents for quicker population coverage may be worth it. Yep, J&J one is the only candidate requiring one shot. Hopefully it will show some good results soon as well.