Distinguishing vaccine efficacy and effectiveness

• Published in: Vaccine Vol. 30; no. 47; pp. 6700 - 6705
• Main Authors: Shim, Eunha, Galvani, Alison P.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier Ltd 19.10.2012; Elsevier Limited
• Subjects: Allergy and Immunology; clinical trials; disease control; disease models; disease transmission; Effectiveness; Efficacy; Humans; Infections; Infectious disease; Infectious diseases; influenza; Influenza Vaccines; Influenza, Human - prevention & control; Mass Vaccination; Mathematical model; Mathematical models; Measles - prevention & control; Measles Vaccine; Models, Theoretical; Ordinary differential equations; Parameterization; Population; Population Dynamics; prediction; protective effect; Smallpox; vaccination; Vaccine; Vaccines; Infectious disease; Efficacy; Vaccine; Effectiveness; Mathematical model; Parameterization
• ISSN: 0264-410X; 1873-2518; 1873-2518
• DOI: 10.1016/j.vaccine.2012.08.045

► There are two mechanisms of vaccine action, ‘all-or-nothing’ and ‘leaky’ vaccines. ► Typical estimation of ‘leaky’ vaccine efficacy has been incorrect in mathematical models. ► We demonstrate how the common measures of vaccine can be correctly estimated. 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. 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. 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. 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.