A characteristic feature of the epidemiology of TBE is that incidence of TBE among humans in disease-endemic regions can vary dramatically from year to year; in the central European countries analyzed in this study, 2-fold to 3-fold annual variations were found. The reasons for these extensive fluctuations are complex and reflect an intricate interplay of factors that control the natural cycle and transmission dynamics of TBE virus. Climatic and ecologic factors influence the population dynamics of ticks and their vertebrate hosts, such as rodents and larger mammals (5
), thereby affecting the abundance of ticks, especially infected ticks, in different years and different regions. A crucial prerequisite for establishing and maintaining TBE virus in its natural cycle seems to be a microclimate that favors the temporal synchrony of larvae and nymph development in spring and that thus enables transstadial transmission of the virus through cofeeding of infected nymphs and uninfected larvae on the same rodent host (29
). These peculiar requirements also help explain why TBE virus is not endemic to many parts of Europe where Lyme borreliosis, which is transmitted by the same ticks, occurs (31
). Even in regions where both agents cocirculate, transmission of Lyme borreliosis seems to follow tick habitats wherever they occur, whereas TBE is confined to only a subset of these locations (9
). In addition to ecologic factors, specific weather conditions and socioeconomic changes can affect persons’ outdoor activities, leading to enhanced or diminished risk for exposure to TBE virus–infected ticks (8
). It is, however, unlikely that the increases and decreases in incidence rates observed from year to year in central Europe result from socioeconomic changes because such changes would not be expected to fluctuate so strongly on an annual basis. Also, similar extents of annual incidence fluctuations are observed in most other countries where TBE virus is endemic (32
), suggesting that these are driven primarily by biological and climatic factors.
In addition to the short-term fluctuations, our data analysis also revealed longer range undulations of incidence rates in intervals of >
5 years, which seemed to be parallel in the 3 countries to a substantial extent (, panel C). Except for the strong overall upsurge of TBE cases in the Czech Republic and Slovenia (but not in Austria) starting around 1992, the long-range incidence curves for 1990–2011 are remarkably similar for all 3 countries, suggesting substantial covariation of the underlying risk-for-exposure conditions. The upsurge of TBE in several European countries around 1993 might result from several factors (8
); however, all factors that influence the dynamics of tick development and human outdoor activities would be expected to have similar effects on the incidence of Lyme borreliosis (5
). Recent data from Latvia, however, clearly demonstrate an uncoupling of the incidence rates for the 2 diseases (33
). The decrease of TBE cases after 1998 was not matched by the increasing incidence of Lyme borreliosis or by the unchanged high activity of ticks (33
). These data, therefore, support the assumption that, in addition to tick vector dynamics and risk for exposure to tick bites in general, the specific factors controlling circulation of TBE virus in its natural hosts (8
) have a major effect on the long-range characteristics of TBE endemicity. A significant increase was not detected in the overall incidence of TBE in Austria in the 40-year observation period (, panel C), in contrast to the upsurges seen in the Czech Republic and Slovenia at the beginning of the 1990s. However, incidence rates among the nonvaccinated population in Austria might be biased toward numbers that are too low because the risk for exposure is probably disproportionately distributed among vaccinated and nonvaccinated persons. Another factor contributing to this phenomenon could be the switch in diagnostic techniques, from hemagglutination inhibition and complement fixation tests to ELISA.
Among all European countries, vaccination coverage is highest in Austria, where ≈85% of the total population have received >
1 doses of the vaccine (24
). This high vaccination coverage has led to a dramatic decline in the overall incidence of TBE in Austria, in stark contrast to neighboring countries to the north (Czech Republic) (11
) and south (Slovenia) (25
), which have relatively low vaccination rates (>
1 doses; data from 2009) of ≈16% and ≈12%, respectively (14
). Similar to what was found in a previous study (24
), the effectiveness of the vaccine for preventing disease is high: 96%–99% after regular vaccination and best-case assumptions. Even among persons with a history of irregular vaccinations, the average protection rate is still >90%. Moreover, vaccine effectiveness is excellent among elderly persons, for whom risk for severe forms of TBE and neuropathologic sequelae is highest (13
). This high protection rate might be associated with the recent finding that the functional quality of antibodies induced by vaccination is independent of age, although the quantity of antibodies is substantially lower in elderly persons (34
). Given the reported data on the immunogenicity of TBE vaccines (22
) in children, the reason for the slightly lower field effectiveness among children is unclear.
In all 3 countries analyzed in this study, incidence rates for TBE were highest among elderly persons, consistent with increased disease severity and concomitantly lower rates of subclinical infections with increasing age (5
). In addition, during the past 10 years, incidence rates have tended to shift toward older age groups. This trend has been especially striking in the Czech Republic (), where elderly persons, compared with those in other countries, were underrepresented from 1990 through 1999 (and for whom incidence rates were lower than for adolescents and young adults); however, in the past decade, incidence of TBE among elderly persons has sharply increased. This development probably reflects the ever-improving situation—including health status—of retired persons, which results in increased physical outdoor activities and, thus, risk for tick exposure (11
). In the Czech Republic and Slovenia, TBE incidence among children and adolescents occurred in distinct peaks, similar to what was observed in Austria during the prevaccination era (26
). This peak has completely disappeared in Austria (), indicating a high degree of protection, especially among groups of young persons, who have the highest risk for exposure. School vaccination programs probably contributed to this achievement. Nevertheless, because vaccination coverage rates (A. Essl, unpub. data) and protection rates () are similar among persons in different age groups in Austria, one would have expected a higher TBE incidence rate among nonvaccinated children in this country during the past 20 years. The factor or factors responsible for the phenomenon observed are currently unknown and might be associated with differences in age-specific risk for exposure in each of the 3 countries. In summary, the example from Austria indicates that TBE vaccination is an excellent way to prevent disease in all age groups. In this country during 2000–2011, vaccination prevented >4,000 cases of TBE.