The first ‘flu’ might have been vaccine related.
Probably not or it would be very mild. The vaccine is not a guarantee. It should create sufficient antibodies but they can’t test every dose. If you get one every year, your chances get much better. That’s why I get one every year (plus it is free and I am cheap).
The new strains are a problem because they are hard to predict. The Hong Kong Flu(1997) leaped over from birds, killing a child in Hong Kong. Swine Flu came over from pigs.