Flu Vaccines:
What I Understand.
Vaccines must be made for specific types of viruses. Influenza has many strains, and a new vaccine must be prepared for each strain. Therefore, scientists must predict which strain will be prevalent in each year, and vaccinate for it. Sometimes they are wrong, sometimes they are right, but either way, you are vaccinated against the strain they prepared for.
What I understand about Vaccines:
Vaccinations are the use of dead or engineered viruses. The same virus that gets us sick is what lets us prevent the sickness, because that virus had been engineered to not be malicious against the human body.
Vaccine - Wikipedia, the free encyclopedia
Influenza vaccine - Wikipedia, the free encyclopedia