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. It needs however to be noted that in the post-COVID-19 pandemic era, the seasonal pattern may be affected by the co-circulation of COVID-19 and/or other factors.

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 declining immunity or changes in the virus characteristics (antigenic drift), or a combination of the two.