The immune system's memory can diminish due to several factors: - Age: Older adults often experience a decline in immune function, known as immunosenescence, which can affect the duration of immunity. - Viral Evolution: Some pathogens, like the influenza virus, mutate frequently, rendering previous immune responses less effective. - Vaccine Type: The nature of the vaccine itself, whether it is live-attenuated, inactivated, or subunit, can influence the longevity of the immune response.
