If 40% of the U. S. population has been vaccinated, then for herd immunity, only about 30% of the population need have the antibody by natural infection. But of course if a large percentage of those vaccinated already had the antibody by natural infection, then the number naturally infected would need be greater than 30%, which almost certainly has happened, about 200,000,000 Americans having been infected during the year-and-a-half long spread of that highly contagious disease.
