Percent of US Population
    
                                  At least  Fully  
                                  one dose  Vaxed  Boosted
    Total                         75.4%     63.3%  39.5%
    Population ≥ 5 Years of Age   80.1%     67.3%  
    Population ≥ 12 Years of Age  85.3%     72.1%  
    Population ≥ 18 Years of Age  87.3%     73.8%  42.6%  
    Population ≥ 65 Years of Age  95%       88.2%  62.7%

https://covid.cdc.gov/covid-data-tracker/#vaccinations_vacc-...

They say 83 million boosters given, which is ~25% of the population. They seem to be using a different denominator for the boosted number there (I guess the number of people eligible for the booster).

It's reasonable to be measuring against eligibility when evaluating progress in administering boosters, but not being eligible for a booster doesn't really make someone less susceptible to infection, so it isn't the right thing to compare across countries.

For reference Ireland • Fully vaccinated: 18+ years: 94.8% 12+ years: 92.8% • Booster / immunocompromised: 18+ years: 67.1%