You may not realize how vaccines are made. From Re-Inventing the Flu Vaccine:
If ya wanna make a flu vaccine, ya gotta break a few eggs. Actually, over a million. The current ‘hen oviduct bioreactor technology’ (a.k.a., using eggs) takes up to nine months in its entirety. That means if health authorities goof in choosing the viral strains they think will be prevalent in the winter, or we have a flu-shot shortage like this year, it’s too late to start a new batch. People get sick; people die.