Current scientific literature suggests that viruses change by small amounts all the time, and every now again (3-5 years for influenza) there is big 'jump' in which a significantly novel strain is identified and characterised. See this excellent review for further explanation:

The evolution of seasonal influenza viruses - Velislava N. Petrova & Colin A. Russell

https://www.nature.com/articles/nrmicro.2017.118