Correct Answer: B. Influenza
Influenza vaccine requires annual strain change because the influenza virus undergoes antigenic drift and antigenic shift, causing continuous evolution of surface antigens (hemagglutinin and neuraminidase). Antigenic drift results from point mutations in viral genes, while antigenic shift involves reassortment of gene segments between human and animal influenza viruses. This genetic plasticity means the circulating strain in any given season differs from the previous year, necessitating reformulation of the vaccine annually. The WHO conducts global surveillance and recommends vaccine composition twice yearly (Northern and Southern hemispheres) based on which strains are predicted to circulate. In India, the seasonal influenza vaccine is updated annually by the IAP (Indian Academy of Pediatrics) and NTEP guidelines. Unlike polio (which uses fixed strains), diphtheria (toxoid-based), or typhoid (polysaccharide or conjugate), influenza's high mutation rate makes it the only routine vaccine requiring strain-specific reformulation each season.
Why the other options are wrong
A. Polio — Polio vaccine (OPV/IPV) uses fixed, attenuated or inactivated strains of poliovirus types 1, 2, and 3. The virus has minimal antigenic variation; vaccine composition remains unchanged year to year. India achieved polio elimination in 2014 under RNTCP using the same vaccine strains, proving stability. This is a trap for students who confuse viral mutation rates. C. Diphtheria — Diphtheria vaccine is a toxoid (inactivated exotoxin), not a live or attenuated organism vaccine. Since it targets the bacterial toxin rather than the organism itself, antigenic drift of Corynebacterium diphtheriae does not affect vaccine efficacy. The toxoid composition remains constant across decades, requiring no strain updates. D. Typhoid — Typhoid vaccines (polysaccharide Vi antigen or Ty21a live attenuated) are based on conserved somatic or flagellar antigens of Salmonella typhi. While some strain variation exists globally, the vaccine does not require annual reformulation. The antigen targets are stable enough that a single vaccine formulation provides cross-protection, unlike influenza's rapid drift.
High-Yield Facts
- Antigenic drift (point mutations) and antigenic shift (gene reassortment) in influenza require annual vaccine reformulation; no other routine vaccine needs this.
- WHO recommends influenza vaccine composition twice yearly (February for Northern Hemisphere, September for Southern Hemisphere) based on surveillance data.
- Polio, diphtheria, and typhoid vaccines use fixed strains or toxoids with minimal antigenic variation and do not require annual updates.
- In India, seasonal influenza vaccine is recommended annually by IAP and NTEP guidelines for high-risk groups (healthcare workers, elderly, immunocompromised).
- Hemagglutinin (H) and neuraminidase (N) are the major surface antigens of influenza; mutations in these genes drive the need for strain-specific vaccines.
Mnemonics
FLU DRIFTS (Influenza requires annual updates) Flu = Low mutation rate? NO. Updates needed? YES. Drift & shift = Reformulation. Immune escape = Frequent changes. Toxoid vaccines = Stable (diphtheria). Use when deciding which vaccine needs annual strain change. FIXED vaccines (Polio, Diphtheria, Typhoid) Fixed strains (Polio OPV/IPV). Inactivated toxoid (Diphtheria). Xenophobic to change (no drift). Each year = same formula. Don't reformulate. Remember: if vaccine is FIXED, strain doesn't change.
NBE Trap
NBE pairs influenza with other viral vaccines (polio) to test whether students understand that viral mutation rate and vaccine target type determine reformulation need. Students may incorrectly assume all viral vaccines require updates, or confuse toxoid-based vaccines (diphtheria) with organism-based ones.
Clinical Pearl
In Indian clinical practice, healthcare workers and elderly patients are counselled annually for influenza vaccination before the monsoon season (June–July), as respiratory infections peak post-monsoon. The vaccine strain changes yearly based on WHO predictions, making it essential to re-vaccinate even previously vaccinated individuals—a key counselling point often missed by clinicians.
_Reference: Jawetz, Melnick & Adelberg's Medical Microbiology Ch. 39 (Orthomyxoviruses); Park's Textbook of Preventive and Social Medicine Ch. 7 (Immunization); Harrison's Principles of Internal Medicine Ch. 195 (Influenza)_