I read this novel around 1985-1990. It was written in French, I cannot tell if it is the original language or a translation.
The story occurs throughout the whole 20th century.
In this novel the world leaders (like JFK, etc.) secretly meet to talk about a big threat that must remain secret. This threat turns out to be immortality.
As far as I remember, the author has put the history in real historical context, i.e. those meetings actually occurred.