Dengue Literature Review

CYD-TDV

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CYD-TDV

Overview

CYD-TDV (trade name: Dengvaxia; developer: Sanofi Pasteur) is the first licensed dengue vaccine and the only one to have completed phase III efficacy trials and received regulatory approval as of 2016. It is a live attenuated tetravalent chimeric vaccine constructed by inserting the prM and E genes of all four DENV serotypes into the genetic backbone of the 17D yellow fever vaccine virus (ChimeriVax platform). Despite its approval in five countries, CYD-TDV carries a significant safety signal: seronegative individuals who receive the vaccine and subsequently become infected experience enhanced disease, consistent with an ADE mechanism — the vaccine effectively primes an immune response that acts like a “first dengue infection” in naïve individuals.

Key Points from Literature

Efficacy

  • Phase III conducted in >30,000 children across five Asian and five American countries
  • Overall vaccine efficacy: 56.5% (Asia trial) and 60.8% (Americas trial)
  • Vaccine efficacy against DHF (severe disease): >80% in both trials
  • Efficacy was substantially higher in seropositive (previously infected) individuals than in seronegative individuals — consistent with the vaccine boosting pre-existing immunity rather than priming protective de novo responses
  • Protects individuals who have had one previous DENV infection and are therefore at risk of ADE-mediated severe secondary disease (see Guzman2016 - Dengue Infection)

Safety signal — seronegative enhanced disease

  • Children ≤5 years at vaccination experienced hospitalised breakthrough DENV disease at 5× the rate of controls
  • Aetiology: placebo and vaccinated children who develop hospitalised dengue during a breakthrough infection are clinically similar, but the underlying mechanism differs — vaccines primed seronegative individuals for ADE when they subsequently experienced their first natural infection
  • The pattern mirrors the maternal antibody ADE mechanism in infants: partial humoral immunity from the vaccine (equivalent to waning maternal antibodies) enhances entry of a subsequent natural DENV infection into Fc receptor-bearing cells
  • This signal was not fully characterised at the time of initial approval but led to subsequent warnings and restriction of use in seronegative individuals (see Guzman2016 - Dengue Infection)

Regulatory Status (as of 2016)

  • Approved in Mexico, Philippines, Brazil (and subsequently other countries)
  • Approved for ages 9–45 years, in populations where at least 70% can be expected to have prior dengue antibodies
  • This age-specific restriction reflects the safety signal: 9-year-olds in endemic countries are far more likely to be seropositive (and therefore to benefit) than younger children

Contradictions & Debates

  • ADE as mechanism of enhanced disease: The enhanced disease in seronegative CYD-TDV vaccinees is widely interpreted as ADE (vaccine-induced non-neutralising antibodies enhancing viral uptake on FcR-bearing cells). This is the most direct clinical validation of the ADE hypothesis in human dengue, but some argue that the mechanism may be more complex — involving T cell responses and original antigenic sin in addition to antibody-mediated enhancement.
  • Pre-vaccination serological screening: After the enhanced disease signal was better characterised, screening before vaccination was recommended, raising practical implementation questions about whether screening tests are sufficiently sensitive and cost-effective in endemic settings.

Sources

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