If I were reading this, I would probably say "nineteen eighteen to nineteen". However, if I were just saying when the flu pandemic was in my own words, I might well say "nineteen eighteen nineteen".
A far more common example is "the nineteen fourteen eighteen war", which is the usual way of speaking the years, in BrE at any rate. This would be written as "the 1914-18 war". However, I cannot quite think how I would read "the 1914-18 war" aloud if I encountered this in print. I'd probably include "to", but I am not sure.
The dash in "1918-19" isn't a hyphen.
