Italian to English Medical Sports / Fitness / Recreation

Ciao a tutti,

I am stuck on this phrase, taken from a college assignment. It's from a webpage containing information on influenza. I'm not sure if I have captured the right meaning though. I found a literal translation to be confusing for target reader, I have a suggested translation below but any suggestions are welcomed!

A presto,

Emily.

tt:
Based on the epidemiology of the influenza, there is a pronounced tendency of all influenza viruses to mutate, that is, changes occur in their surface proteins which allow them to evade the host’s immune response, a barrier of defence formed by the population who have previously had the influenza infection.

ST:

Alla base della epidemiologia dell'influenza vi è la marcata tendenza di tutti i virus influenzali a variare, cioè ad acquisire cambiamenti nelle proteine di superficie che permettono loro di aggirare la barriera costituita dalla immunità presente nella popolazione che in passato ha subito l’infezione influenzale.