Sočan et al. BMC Public Health 2010, 10:360 http://www.biomedcentral.com/1471-2458/10/360 Page 4 of 6 Figure 1 Observed vs predicted VZV seroprevalence rates for Slovenian data (predictions generated by the parametric model; see text). Observed rates are shown in black dots along with 95% binomial confidence intervals. The numbers below/above the CI lines represent the total number of subjects in each age group. Proportions of seropositives predicted by the model are shown in solid line. The dashed line is the model-derived force of infection curve (the corresponding scale is on the right; lmax 0.282 at 5.0 years). receive the vaccine. The annual vaccination report data (electronic dataset, National Institute of Public Health) show that less than 100 people are vaccinated against varicella each year. Due to the low number of vaccines, these vaccinations have no impact on the national varicella epidemiology or seroepidemiology. Introduction of the varicella vac- cine to the national childhood immunisation programme has already been under consideration for some time. The serosurvey presented in this paper has been con- ducted to elucidate the seroepidemiology in Slovenia in the prevaccination period. As the tested sera were resi- dues of specimens taken for routine diagnostic tests, they do not represent a random sample, which is an inherent weakness of many seroprevalence studies. However, varicella is so prevalent a disease that the esti- mates of seropositivity based on either systematically collected or residual samples yield similar results [35]. Table 2 Parameter estimates of the fitted parametric model [equation 6, Additional file 1] Parameter a b Value 0.153 0.199 SE 0.012 0.016 In our study, 80% of infants aged 4 months or less were VZV seropositive. Our results reconfirm the known fact about newborn infants receiving maternal VZV antibodies. This passive neonatal immunity is of a short duration as the half-life of passively acquired maternal immunoglobulins is only 6 weeks [36]. The gradual loss of maternal antibodies has already been documented for other infectious diseases, including measles, mumps and rubella. The rapid loss of protec- tion renders children highly susceptible to various infec- tious diseases including varicella [1,36]. In Slovenia, the duration of maternity leave is 12 months (one month before the expected date of birth and 11 months after- wards) and children enter kindergarten from one year of age onward. Therefore, infants’ siblings are probably the most common source of infections in the first year of life. In our study, only 16% of infants from 5 to 12 months of age had detectable antiVZV antibodies; most probably these were acquired by natural infection. Other studies have reported similar proportions of seropositive children in this age group [1,36]. In Slovenia, the percentage of children attending kin- dergarten is constantly rising. In the past, the increase in the number of children attending kindergarten was observed mainly among children from three years on