Vaccine efficacy is not uniform across population sub-groups. Examine this statement in light of concerns raised over Qdenga's performance in seronegative individuals.

Q. Vaccine efficacy is not uniform across population sub-groups. Examine this statement in light of concerns raised over Qdenga's performance in seronegative individuals. (15 marks, 250-350 words)

Vaccine efficacy is a population average, not an individual guarantee — it varies with age, immune history and pathogen strain. The CDSCO's 2026 approval of Qdenga (TAK-003), India's first dengue vaccine, illustrates this sharply, since its protection depends on whether the recipient has been infected before [1].

How sub-group variation arises in Qdenga - Prior serostatus is decisive: in seropositive persons (previously infected), efficacy was demonstrated against all four serotypes, while in seronegative recipients protection appeared against DENV-1 and DENV-2 but not against DENV-3 [2]. - Serotype dependence: Qdenga is live-attenuated with a DENV-2 genomic backbone, so immunity is strongest against DENV-2 and weakest against DENV-3/DENV-4 [1][2]. - Age: WHO recommends its use in children aged 6–16 years in high-transmission settings, whereas India licensed it for 4–60 years — a wider band than the strongest evidence base [1][3].

Why the seronegative concern matters in India - All four serotypes circulate in India, with lakhs of reported cases annually, so a seronegative child vaccinated in a DENV-3-dominant district may gain little benefit [4]. - The theoretical risk of antibody-dependent enhancement — a partial immune response worsening a later infection — is the lesson of the earlier Dengvaxia experience.

Regulatory response - CDSCO granted approval after evaluation under the Drugs and Cosmetics Act, 1940, and permits use without pre-vaccination screening, easing rollout [1]. - WHO prequalification (May 2024) and licensure in over 40 countries provide external assurance [3].

Sub-group heterogeneity therefore calls not for rejecting the vaccine but for calibrating its use. Prioritising high-burden, high-seroprevalence districts, sustaining serotype surveillance under the NCVBDC, and rigorously executing the mandated post-marketing safety study would convert an uneven tool into an effective one. Read with sustained vector control, Qdenga can advance the SDG-3 goal of ending epidemics of neglected tropical diseases.

(~330 words)

Sources: 1. PIB — CDSCO Approves India's First Dengue Vaccine, Strengthening National Dengue Prevention Efforts — approval, DENV-2 backbone, 4–60 years, no pre-vaccination screening, statutory basis 2. Long-term efficacy and safety of a tetravalent dengue vaccine (TAK-003): 4·5-year results from a phase 3 trial, The Lancet Global Health — serostatus- and serotype-specific efficacy, lack of efficacy against DENV-3 in seronegatives 3. WHO position paper on dengue vaccines – May 2024 — WHO age recommendation and prequalification 4. NCVBDC, Ministry of Health & Family Welfare — Dengue Situation in India — national dengue case burden and surveillance framework