Effectiveness and Timing of Vaccination during School Measles Outbreak

Axel Antonio Bonačić Marinović

, Corien Swaan, Ole Wichmann, Jim van Steenbergen, and Mirjam Kretzschmar

Author affiliations: National Institute for Public Health and the Environment (RIVM), Bilthoven, the Netherlands (A.A. Bonačić Marinović, C. Swaan, J. van Steenbergen, M. Kretzschmar); University Medical Centre Utrecht, Utrecht, the Netherlands (A.A. Bonačić Marinović, M. Kretzschmar); Robert Koch Institute, Berlin, Germany (O. Wichmann); and Leiden University Medical Centre, Leiden, the Netherlands (J. van Steenbergen)

Abstract

Despite high vaccination coverage in most European countries, large community outbreaks of measles do occur, normally clustered around schools and resulting from suboptimal vaccination coverage. To determine whether or when it is worth implementing outbreak-response vaccination campaigns in schools, we used stochastic outbreak models to reproduce a public school outbreak in Germany, where no vaccination campaign was implemented. We assumed 2 scenarios covering the baseline vaccination ratio range (91.3%–94.3%) estimated for that school and computed outbreaks assuming various vaccination delays. In one scenario, reacting (i.e., implementing outbreak-response vaccination campaigns) within 12–24 days avoided large outbreaks and reacting within 50 days reduced outbreak size. In the other scenario, reacting within 6–14 days avoided large outbreaks and reacting within 40 days reduced the outbreak size. These are realistic time frames for implementing school outbreak response vaccination campaigns. High baseline vaccination ratios extended the time needed for effective response.

Measles is a highly contagious disease that causes illness and death in developing and industrialized countries. Measles has an estimated basic reproduction number (R₀) range of 12–40 (1–5), meaning that 1 case introduced into a susceptible (naive) population will produce, on average, that number of secondary cases. Worldwide, measles is the main vaccine-preventable cause of death among children; >31 million cases occur every year, and case-fatality rates for industrialized countries are ≈0.1%–0.2% (6). In 2001, the World Health Organization (WHO) and the United Nations published a global strategic plan for measles (7); the aims of the plan are to sustainably reduce deaths from measles and to interrupt measles virus transmission in countries and regions with elimination objectives.
In 2002, the WHO Region of the Americas was declared free from endemic measles transmission, which was achieved by implementing immunization programs with very high vaccination coverage (>95%). This goal has not been achieved in the WHO European region, for which the target year for measles elimination was 2010. Measles is so highly contagious that the average vaccination coverage in Europe (80%–95%) (8) is not high enough to prevent outbreaks among nonvaccinated persons. The new target year for measles elimination in the WHO European region is 2015 (9).
The only means of protection against measles are prior infection or vaccination. Several studies have focused on the effectiveness of mass outbreak-response vaccination campaigns for controlling measles outbreaks in settings where incidence and morbidity and mortality rates are high. Although some studies suggest that mass outbreak-response vaccination campaigns will not stop measles epidemics because of the rapid spread of the disease (10–12), other studies, which used more recent data, show that outbreak-response vaccination campaigns can successfully reduce illness and death (13–17). The current WHO guidelines recommend mass outbreak-response vaccination campaigns when a measles outbreak is confirmed in settings with a goal of reducing deaths from measles (18) and where most measles cases occur in children <5 a="a" age.="age." age="age" and="and" are="are" as="as" baseline="baseline" because="because" campaigns="campaigns" cases="cases" children="children" distribution="distribution" european="european" for="for" high="high" however="however" href="http://wwwnc.cdc.gov/eid/article/18/9/11-1578_article.htm#r19" implementing="implementing" in="in" incidence="incidence" is="is" low="low" measles="measles" morbidity="morbidity" mortality="mortality" no="no" nonvaccinated="nonvaccinated" noted="noted" of="of" often="often" older="older" outbreak-response="outbreak-response" rates="rates" ratio="ratio" recommendation="recommendation" region.="region." school="school" settings="settings" shift="shift" such="such" the="the" there="there" these="these" title="19" toward="toward" vaccination="vaccination" where="where" who="who" years="years">19

). Consequently, outbreaks initially spread within relatively closed populations of children, such as schools or daycare centers, because of the high rate of social contact among nonvaccinated children in these establishments. Therefore, we focused our study on outbreak-response vaccination campaigns that targeted establishments with children where a measles outbreak was occurring in settings with high BVRs. A delay from detection of an outbreak to implementation of an outbreak-response vaccination campaign to onset of an effective immune response of those vaccinated is inevitable. Because measles is highly contagious, many children might become infected during that delay. Thus, whether vaccinating children against measles during a school outbreak would substantially affect the outcome of a newly forming epidemic is in doubt (20). In this study, we used stochastic models to estimate the expected size of an outbreak in a school, depending on the delay between detection and implementation of complete school outbreak-response vaccination campaign.