Definitely, there is selection pressure for new strains so that it can reinfect people which have immunity from last year. The flu virus is really good in muta

Definitely, there is selection pressure for new strains so that it can reinfect people which have immunity from last year. The flu virus is really good in mutating because its genome is made 7 or 8 parts which can be exchanged between strains. Most other viruses don't have these segmented genomes and are somehow stuck in a local optimum, so they can't evade the immunity so easily.