Timing of influenza vaccination
It takes 10 to 14 days following vaccination, before an immune response and protection develops. Therefore, most countries start immunisation in the early autumn. It is rare that the annual influenza epidemic season starts before mid-November, and the season commonly runs to the end of May the following year.
The protection provided by the vaccine is expected to last for at least one influenza season, but it does wane with time. It is currently not clear if this is mainly due to waning immunity or virus drift, or a combination of the two.