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Measuring and Observing Positive And Negative Externalities Caused By Vaccines: Do We Have The Right Assessment Approach Available?

      Objectives

      Vaccination not only Results in direct protection to those being vaccinated, it also has the potential of inducing indirect protection among unvaccinated individuals (=herd protection or positive externality). However, negative unintended consequences or externalities may also result from vaccination programmes (e. g. rebound effects). It is our purpose to present how and when these positive and negative externalities can be observed and measured.

      Methods

      We first identify under what conditions herd protection is most likely to occur. We then explore how negative rebound effects can also be manifested. Detailed illustrations of both positive and negative effects are presented for different infections in relation to mass vaccination programmes. Lastly, we discuss Methods for observing and measuring these externalities.

      Results

      Optimal herd protection is likely to be observed when the vaccine has a high quality-induced immunity, substantial effect on the force of infection, and appropriate vaccine coverage and distribution. Example: HPV vaccination of 12-16 year old girls resulted in a 50% decrease of anogenital warts in 15-19 year aged adolescents in Denmark observed over a 4-year period. Rebound effects may potentially occur due to vaccine-related age shifting, decreased natural immunity, serotype replacement, low-medium coverage and non-homogeneous vaccine distribution. Example: increased herpes zoster incidence in elderly post-varicella vaccine introduction. Those externalities can be captured through observational studies using real-life data, or may be estimated using dynamic transmission modelling techniques.

      Conclusions

      Limitations are inherent in those studies and involve substantial ambiguity in the process of observing and quantifying the indirect effects, making accurate evaluation troublesome. However the nature of these outcomes could be critical for achieving good economic value when decision- makers are evaluating a novel vaccine for introduction into a particular region or people group. More investigation is needed to identify and develop successful assessment methodologies for precisely analysing these outcomes.