Protective immunity is typically not long-lasting against which of the following infections, largely due to frequent antigenic variation?

Introduction / Context:
Duration of immunity varies by pathogen. Influenza viruses undergo antigenic drift (and occasionally shift), leading to changes in surface antigens hemagglutinin and neuraminidase. As a result, prior immunity may provide limited protection, necessitating periodic vaccine updates.

Given Data / Assumptions:

• We compare typical durability of immunity after infection or vaccination.
• Focus on pathogens known for antigenic change.

Concept / Approach:
Antigenic drift results from point mutations altering epitopes; antigenic shift (reassortment) can produce novel subtypes. Consequently, long-term sterilizing immunity is uncommon, and seasonal vaccines are reformulated annually to match circulating strains.

Step-by-Step Solution:

Verification / Alternative check:
Public health recommendations call for annual influenza vaccination, confirming limited durability of strain-specific immunity.

Why Other Options Are Wrong:

• Pertussis: Immunity wanes over years, but the classic exam answer for “not long-lasting due to frequent antigenic variation” is influenza.
• Diphtheria and mumps: Vaccination confers durable protection in most individuals with boosters as scheduled; antigenic drift is not a dominant issue.
• Rubella: Generally durable vaccine-induced immunity.

Common Pitfalls:

• Equating waning immunity from lack of boosters with true antigenic variation.

Final Answer:
Influenza