The original scientific definition was that herd immunity was achieved when the number of people surviving outpace the numbers of who died.
(Also, when herd immunity is achieved there is no longer a pandemic, which happened the first week of April of 2020.)
That definition is the one every doctor was taught in medical school.
The Who changed it to "when everyone is vaccinated."
It is good to remember that the head of WHO is not even a medical doctor.
Can you explain where you found that definition? It does not match ANY definition of herd immunity that I have ever encountered in a career of dealing with infectious disease.
Herd immunity is a function of how contagious a disease is, which is expressed with the R-naught (R0) value. It is determined by the equation "1-(1/R0)." Covid-19 has an R0 value of ~2.5; this means the necessary level of herd immunity to inhibit disease spread is 1-1/2.5=0.6, or 60%.
FYI, the R0 value of 2.5 means that, on average, an infected person spreads the disease to 2.5 other people.
“The WHO changed the definition of herd immunity last summer....
The Who changed it to “when everyone is vaccinated.”
So, was it Pete Townshend or was it Roger Daltry that changed it? ;-)