The bit that's not quite right is some of the info in the flu vaccine.
It's not single strain, it's about 4 strains - those which the epidemiologists think wil, be the dominant ones in the next flu season (lots of number crunching and modelling goes in to that, and it's done globally so includes what strains are emerging in the Southern Hemisphere and the likelihood of transmission to the North.
They usually get it right, and the number of flu cases is cut significantly. A couple of years ago, the predictions were wrong, so the jab didn't suppress the number of cases of flu.
So yes, as it's always working on prediction - even though historically the prediction is normally right - it's not surefire as it is for diseases which do not mutate rapidly like flu.
It's only given to two second of the population at the moment, the 'superspreaders' (ie children) because if it's not so much in schools the overall spread is reduced considerably, and the vulnerable (elderly people, the pregnant, and those with certain underlying health conditions) to keep the death rates down and reduce the number of hospitalisations