Herd immunity only works if the herd is immune. If only a portion of the herd is immune, the rest of the herd will spread disease.
It requires a critical mass to work, and it protects those who can not get the vaccine for medical reasons (like allergic reactions, etc.)
I don’t know that they sound that different, but I definitely “pronounce” them differently in that my tongue is in a different party of my mouth for both of them. When I say clothes, my tongue is near touching my front teeth, where as close is more just below that ridge behind my teeth, so farther back.
I’m from the center of the U.S. for reference.