Final answer:
New Influenza strains evade the immune system by altering their hemagglutinin and neuraminidase proteins, which are the targets of preexisting immunity. This leads to the need for annual updates of the influenza vaccine.
Step-by-step explanation:
New viral strains of Influenza lack recognition by the immune system that was primed by previous encounters with the virus. These strains undergo changes in their surface proteins, namely the hemagglutinin and neuraminidase, through processes known as antigenic drift and shift. Such changes enable the virus to evade preexisting immunity in the population, potentially leading to epidemics. Influenza A viruses are highly virulent and can cause seasonal pandemics, while Influenza B and C are less virulent, with Influenza C causing the mildest symptoms and being least associated with epidemics. To combat this, vaccines are updated annually to include the most prevalent strains, providing protection against the current seasonal viruses.
Influenza viruses are known for their ability to rapidly mutate through antigenic drift and antigenic shift, making it difficult for the immune system to recognize different strains. As a result, new viral strains of Influenza lack recognition by the immune system.