That's not how herd immunity works.
Herd immunity means that enough people are immune that it's highly, highly unlikely for an infected person to transmit the infection to a non-immune person.
The percentage of the population required for that depends on how contagious the infection is (the R0); something like measles has a high R0 or around 12-18, meaning that 1 infected person can expect to introduce the virus to 12-18 other people. So herd immunity means a high enough proportion of the population has immunity that none of those 12-18 people are non-immune; which is somewhere around 95%.
The R0 for COVID-19 is though to be 2-3, so each infected person is likely to introduce the virus to 2-3 other people; so you only need 60-70% of the population to have immunity for herd immunity.
Herd immunity doesn't mean that no-one gets it, and everyone is immune; it means that those who can't risk gaining immunity are highly unlikely to ever come into contact with the infection.
Once herd immunity is gained, the disease is not eradicated; just extremely rare. Once herd immunity is lost, the disease can become rampant - at least in pockets (again, see the example of measles where herd immunity had been obtained, until Andrew Wakefield fucked up, and now it has been lost in some segments of the population, and we now get flare-ups)