Sočan et al. BMC Public Health 2010, 10:360 http://www.biomedcentral.com/1471-2458/10/360 and up to entering elementary school [37]. In the last few years, however, the number of children attending kindergarten has also been increasing in the age group of 1-3 years. According to national statistics, slightly more than two thirds of preschool children attended kindergarten in the school year 2008/09. The percentage of preschool children aged less than 3 years and 3-6 years who attended kindergarten was 44% and 82%, respectively. The high proportion of children having out-of-home social contacts in the first years of life may partially explain the rapid rise in VZV seropositivity. Approximately 70% of children had VZV antibodies before they entered their first year of primary education. The proportion of seropositive preschool children in our study is similar to those observed in other countries with a temperate climate. According to the ESEN2 study, over 50% of young children had antibodies to VZV by 5 years of age, with few exceptions - there were less then 40% of seropositive children in Italy and more then 80% in the Netherlands and Belgium [16]. In some countries, the infants are cared for in nurseries and other child-care facilities [16]. The early commencement of group childcare probably influences the seropositivity rate in a geographic region. European countries collect data on vaccine-preventa- ble diseases through paediatric sentinel systems or, less frequently, through obligatory reporting systems. Slove- nia is among those rare European countries where vari- cella has been a mandatory notifiable disease for over 20 years. As many as 75% of varicella cases reported were pre-school children, the age group of 3 to 4 years being the most affected. Age-specific distribution of var- icella cases showed a downward trend for all age groups except for children younger than 4 years. In this age group, the incidence rate demonstrated an evident upward trend [38]. However, these changes could also theoretically derive from modification in reporting - e.g. a general increase in under-reporting over time. The obligatory notification system has been assessed for completeness and consistency recently [39]. The average rate of notified cases was 76.2% in last ten years when compared to health statistics data. The mandatory noti- fication system in Slovenia provides enough information to survey age/sex-specific varicella trends in the prevac- cine era [39]. Kindergarten attendance is probably one of the social determinants for the high number of vari- cella cases and high seroprevalence in preschool chil- dren in Slovenia. The number of siblings also determines varicella susceptibility. One study showed that individuals growing up without a sibling or with one sibling were most likely to be seronegative [40]. In Slovenia, the low number of children per family (37% of families having only one child) could have a negative effect on varicella seropositivity. Page 5 of 6 There is a lack of varicella immunity in teenagers, and a small proportion of young adults, aged from 19 to 34 years, are still seronegative. The decreasing proportion of seronegatives (from 5.9% to 3.2%) in this age group shows that infection (most probably associated with a clinically apparent disease) still occurs, although the likelihood of coming in contact with VZV is decreasing. In our study, the highest force of infection (FOI) has been observed in the 6 to 11 or the 5 to 9 years groups (0.361 and 0.391, respectively), estimated by the semi- parametric method. The parametric method identified the highest force of infection (0.282) at age 5 (Figure 1). The VZV FOI in Slovenia is comparable to Germany, Finland and Spain for all three age groups (<5, 5-9, >10 years) [16]. Compared to our results, the FOI for chil- dren <5 was much higher in the Netherlands, Belgium and Luxembourg. The preschool mixing pattern is the major determinant for a high FOI. A study compared two groups of children - a group that entered all-day nurseries or kindergartens early and a group that entered late [41]. The FOI in the first group was much higher than in the latter. The social behavior and cli- matic determinants deserve detailed study in the future to improve our understanding of varicella epidemiology in the prevaccination and postvaccination periods. Conclusion In conclusion, our data show that the most appropriate approach to reduce the burden of varicella would be a universal, standard two dose varicella vaccination pro- gramme (the first dose administered at 12-18 months and the second at 3-6 years) [20]. After the introduction of varicella vaccine in the childhood immunisation sche- dule, a vigilant surveillance of varicella epidemiology and seroepidemiology should continue in order to promptly identify gaps and unexpected epidemiological changes in both varicella and herpes zoster. Additional material Additional file 1: Modelling of force of VZV infection. Additional file 1 contains a detailed description of semiparametric and parametric estimation of force of VZV infection. Author details 1Centre for Communicable Diseases, National Institute of Public Health of Republic of Slovenia, Trubarjeva 2, 1000 Ljubljana, Slovenia. 2Laboratory for virology, Centre for Communicable Diseases, National Institute of Public Health of Republic of Slovenia, Bohoričeva 15, 1000 Ljubljana, Slovenia. 3Rho Sigma Research & Statistics, Topniška 45, 1000 Ljubljana, Slovenia. Authors’ contributions MS conceived and designed the study, organised serological data collection, managed the overall project coordination and drafted the major portion of the manuscript. NB participated in the serological analysis and interpretation