3 months, maybe 6, then they'll have the vaccine in widespread production. Flu vaccine is easy, but predicting which ones to make isn't.
According to
this, it's not easy (takes an extra step to adapt the various flu strains to grow in chicken eggs) or quick (ordering the eggs has a several month lead time).
Witness the current shortage. If it was easy to make, it would be easy to replace.
Containment seems like a more effective option, but it'll have implications for travel and shipping and....