Elsevier

Vaccine

Volume 26, Issue 21, 19 May 2008, Pages 2580-2588
Vaccine

Behavioural responses to perceived risk of tick-borne encephalitis: Vaccination and avoidance in the Baltics and Slovenia

https://doi.org/10.1016/j.vaccine.2008.03.029Get rights and content

Summary

Tick-borne encephalitis (TBE) incidence increased markedly in the Baltics and Slovenia in the early 1990s, but then declined again in some places. Our analyses of temporal and spatial data on TBE incidence and vaccination revealed that over 1970–2005 up-take of vaccination varied in both time and space according to incidence, i.e. was apparently responsive to perceived risk. Since 1999, however, decreases in incidence in many counties within each country have far exceeded vaccination rates or immunity through natural exposure, and in Latvia and Lithuania these changes are correlated with previous incidence. Survey data on human activities in Latvia revealed that people in socio-economic groups whose behaviour put them at highest risk of exposure to ticks in forests, including people with lower education and lowest incomes, are least likely to be vaccinated. We conclude that risk avoidance through changing human behaviour has driven incidence-dependent decreases in TBE infection, but targeted vaccination campaigns could provide more secure protection.

Introduction

Epidemiologists increasingly recognize that human behavioural responses to perceived risk of infection are important in determining epidemic patterns [1]. The viral zoonosis tick-borne encephalitis (TBE) is an excellent system within which to explore this because exposure to infected ticks, mainly in forest habitats, can be reduced or avoided. Alternatively, safe and up to 96–99% effective vaccines may be deployed [2]: in Europe ‘FSME Immun™’ is produced by Baxter [formerly Immuno] using an Austrian isolate (strain Neudoerfl), and ‘Encepur™’ is produced by Novartis [formerly Chiron, formerly Behring] using a German isolate (strain K23), and a Russian vaccine is produced by Virion, Tomsk and the Institute of Polyomyelitis and Viral Encephalitis, Moscow. Nevertheless, TBE incidence increased markedly in most countries during the 1990s. With no specific treatment, it is one of the most serious vector-borne infections of humans in many parts of Europe, Russia and some parts of northern Asia; the Western virus subtype in Europe causes a ca. 1% case fatality rate, long recovery processes, neurological sequelae and severely decreased quality of life [3], [4].

For vector-borne zoonoses, where humans do not play a role as natural amplifying hosts, there is a simple relationship between the percent vaccination in the human population and the percent reduction in incidence of infection as long as conditions are homogenous and risk factors do not change. This is well illustrated in Austria (Fig. 1) where ca. 85% reduction in annual TBE cases from ca. 300–700 in 1975–1982 to 40–100 in 1998–2005 was achieved by improving the vaccination coverage of the at-risk population to nearly 90% [2]. This is exceptional; vaccination in other European countries, including Switzerland, Sweden, and the Czech Republic, rarely exceeds ca. 10% [2], [5], [6], [7]. Populations, however, are rarely homogenous. The public health impact of even a modest degree of vaccination will be disproportionately increased if it is targeted correctly at people who are at high risk due to geographic location or work and leisure activities. Conversely, vaccinating people who are not at risk is wasteful and may also undermine confidence in the vaccine if the population impact is less than predicted. Knowing who is most at risk and who is least likely to use vaccines will improve targeting, and also throw light on behavioural responses to perceived risk.

In the Baltic States (Estonia, Latvia and Lithuania) and Slovenia, TBE incidence showed a particularly marked, but spatially variable, upsurge to reach the highest incidence levels (18–54 cases per 100,000 population) in Europe by the mid 1990s [8], [9]. In some areas, incidence then declined equally sharply. In this paper we examine the reciprocal relationships, both temporal and spatial, between vaccination up-take and TBE incidence in these countries. We also analyse socio-economic indicators of individual behaviour associated with exposure to ticks, and of the probability of being vaccinated. Altogether, the results highlight the influence of human behaviour in determining, and thereby potentially avoiding, the risk of infection.

Section snippets

TBE incidence and vaccination

Annual case numbers of TBE in each ‘county’ (admin level 1) of Estonia, Latvia, Lithuania and Slovenia, based on serological confirmation of all notified cases, mandatory from the 1970s to the present, were provided by each national Public Health Institute. These were converted to incidence per 100,000 of the population.

The same Public Health Institutes also provided official data, based on doctors’ returns, on the number of people vaccinated against TBE, or the number of vaccination doses

Vaccination rates: variation in time

Official statistics show an increase in the annual reported numbers of completed vaccination courses (third primary doses) from the mid 1990s that follows the increase in TBE incidence (Fig. 2). This is especially clear in Estonia and Latvia, where vaccination records over >30 years span the period of sharp increases in both TBE incidence and vaccination that coincided with political independence. During Soviet rule, when the state was responsible for immunisation against TBE, apart from some

Conclusions

TBE epidemiology is a highly dynamic system, with increases and decreases due to an increasingly understood and quantified proposed nexus of biological and socio-economic factors [28]. In the Baltic States and Slovenia, both the number of TBE cases and the number of vaccinations increased during the 1990s, the latter apparently in response to the former, presumably due to increasing perceived risk. In Latvia, however, after the peak in the 1990s, TBE incidence decreased markedly in almost all

Acknowledgements

Data for this study were supplied by the Health Protection Inspectorate and National Institute for Health Development in Estonia, the State Public Health Agency in Latvia, the Centre for Communicable Diseases Prevention and Control in Lithuania and the Communicable Disease Centre, Institute of Public Health of the Republic of Slovenia, Ljubljana, Slovenia. This study was funded by the Wellcome Trust (grant no. 070696/Z/03/Z), the Estonian Science Foundation (grant 5963) and EU grant

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