DISTINGUISHING VACCINE EFFICACY AND EFFECTIVENESS

Tuesday, 21st of October 2014

[source]Vaccine[|source]

The interchangeable use of terms used to measure and parameterize vaccine efficacy and effectiveness can lead to inaccurate parameterization of epidemiological models and needs to be made explicit. Vaccine efficacymeasures the protective effects of vaccination by the reduction in the infection risk of a vaccinated individual relative to that of a susceptible, unvaccinated individual.  In contrast, vaccine effectiveness can be further categorized into the  direct ,  indirect ,  total  and  overall  impact of the vaccine. Direct effects compares the direct risk of a randomly selected individual with and without the vaccination program. Indirect effects can be estimated from the difference in the degree of protection that unvaccinated individuals receive in the presence versus the absence of a vaccine program. Total effectiveness of vaccination is the effect of the vaccination program combined with the effect of the person having been vaccinated.  Overall  effectiveness of a vaccination program is defined as the reduction in the transmission rate for an average individual in a population with a vaccination program at a given level of coverage compared to an average individual in a comparable population with no vaccination program. 

In this report, the authors demonstrate how the vaccine efficacy as well as the four common measures of vaccine effectiveness can be correctly estimated from typical attack rate data for influenza and measles, and determine the threshold vaccine coverage required to attain a specific level of effectiveness for each measure. Detailed mathematical model and the potential pitfalls in the parameterization of models and the resulting underestimation of vaccination effects are available at: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3798059/

 

ABSTRACT

BACKGROUND: Mathematical models of disease transmission and vaccination typically assume that protective vaccine efficacy (i.e. the relative reduction in the transmission rate among vaccinated individuals) is equivalent to direct effectiveness of vaccine. This assumption has not been evaluated.

METHODS: We used dynamic epidemiological models of influenza and measles vaccines to evaluate the common measures of vaccine effectiveness in terms of both the protection of individuals and disease control within populations. We determined how vaccine-mediated reductions in attack rates translate into vaccine efficacy as well as into the common population measures of  direct ,  indirect ,  total , and  overall  effects of vaccination with examples of compartmental models of influenza and measles vaccination.

RESULTS: We found that the typical parameterization of vaccine efficacy using direct effectiveness of vaccine can lead to the underestimation of the impact of vaccine. Such underestimation occurs when the vaccine is assumed to offer partial protection to every vaccinated person, and becomes worse when the level of vaccine coverage is low. Nevertheless, estimates of  total ,  indirect  and  overall  effectiveness increase with vaccination coverage in the population. Furthermore, we show how the measures of vaccine efficacy and vaccine effectiveness can be correctly calculated.

CONCLUSIONS: Typical parameterization of vaccine efficacy in mathematical models may underestimate the actual protective effect of the vaccine, resulting in discordance between the actual effects of vaccination at the population level and predictions made by models. This work shows how models can be correctly parameterized from clinical trial data.