Israel is definitely the place we'll see the first big impact (or at least we all really hope so). They are already well past 30% with the first shot. I was kind of nervous last week, when they were the leading nation in the world at getting vaccinated, and simultaneously shooting up to new record highs of new cases. But, this week we are just starting to see the first signs of a beneficial impact. This would suggest that you need something like 30% of population with one shot (depending on how many have already been infected, of course) to see a slowdown.

The U.S. is currently at 5%, and we're actually one of the countries furthest along.

At 30% with first shot they're probably half way there to effective herd immunity. One thing I read out of Israel was vaccinating the most venerable would bring the death rate down dramatically. Something like a 80/20 applies.

What I'm worried about is a surge in infections in the gap between population wide herd immunity and just most the venerable. As the death rate falls people will get really careless.

The confirmed cases in Israel is over 6% of the population, and the multiple of that which would include asymptomatic and others who just didn't get tested is unknown, but probably somewhere from 4-8. Even if it's as the low end of that range, you could already be halfway to herd immunity. Of course, the ones getting vaccinated may often already have had it (whether they know it or not).

Since the death rate tends to lag the confirmed cases rate by a couple weeks, I think the deaths will still go up for a bit regardless of whether or not people get careless.