Sočan et al. BMC Public Health 2010, 10:360 http://www.biomedcentral.com/1471-2458/10/360 force of infection in the early months of life. Exploratory analyses with our data showed the age group <0.4 yr (~5 months) to be associated with practically 100% sero- positivity. This is in accordance with the general notion of the average maternal antibody protection of about 6 months. To avoid this confounding factor, data from infants <6 months of age (N = 73) were excluded from further analysis. The second issue is brought about by the fact that our data do not represent a random sample from the population. This is not an uncommon feature of sero- epidemiological studies, but the issue should be taken into consideration when interpreting the results [16]. Results Serological testing Overall, 3,158 of 3,689 (85.6%) sera tested positive for varicella IgG antibody; the percentage of positive results was similar between males and females (83.2% vs. 87.5%, respectively). The positivity rate for varicella antibodies was high in the first months of life - IgG antibodies were detected in 81% of children less than 6 months of age. This pas- sively acquired immunity, however, did not persist long and the percentage of IgG-positive children was only 16% in the age group 6-12 months. Presumably some children in this age group have already acquired natural varicella immunity. By their fifth year of age, more then half (56.2%) of the children tested seropositive for VZV. Seropositivity increased with increasing age and, at 8 years of age, VZV antibodies were detected in 90% of children. The rise in seropositivity was moderate in teenagers. The trend was similar for males and females. There was still a 4% seronegative group of individuals in young adulthood. Also, there was a relatively large proportion (2.8%) of seronegative subjects among women of childbearing age (15-49 years). Semiparametric estimation of force of VZV infection Table 1 shows the estimates of the force of infection (l) for the Slovenian sample using two different age group- ings: (a) 0.5- < 6, 6-11 and ≥12 years, and (b) 0.5- < 5, 5-9 and ≥10 years. In both cases the limits were selected to allow a direct comparison of our results with the results of two recent reports [16,27]. Among the limitations of the model, assuming a pie- cewise constant force of infection for the different age groups, is that it may yield unrealistic results for certain age groups. For example, with our data it yielded a value of l = 0 in the age group 12-18 years (grouping used: <3, 3-6, 6-12, 12-18, 18-26, >26 years), thus repro- ducing the problem reported by Ogunjimi [34]. There- fore, a parametric non-linear method was employed as an additional means of estimation. Table 1 Estimates of force of infection l (and 95% CI) in the Slovenian sample with two age groupings: (a) 0.5- < 6, 6-11 and ≥12 years; (b) 0.5- < 5, 5-9 and ≥10 years Age grouping (group limits) a (6, 12 yrs) b (5, 10 yrs) l l1 0.182 l2 0.367 l3 0.008 l1 0.175 l2 0.391 l3 0.025 Page 3 of 6 95% CI 0.158-0.206 0.285-0.448 0-0.032 0.147-0.202 0.303-0.480 0.003-0.046 Parametric estimation of force of VZV infection The parameters of the model were estimated by the non-linear least squares method using the R statistical package, and the goodness-of-fit was evaluated by calcu- lating the deviance. The overall fitting of the parametric model is good (deviance 0.031, residual standard error 0.035 on 25 d.f.), which is also supported by Figure 1. This allows for the model-based estimation of the force of infection, as the hazard of infection can be calculated as a negative logarithm of the derivative of the survival function yielding the following formula: (x) = axe −bx The formula comes from a procedure of model selec- tion (equation 6, Additional file 1). From the practical standpoint, its parameters can be interpreted as deter- mining the maximum force of infection (parameter a) and the age at the maximum force of infection (recipro- cal of parameter b). A graphical representation of the data and the fitted force of infection curve are shown in Figure 1 and the parameter estimates are presented in Table 2. Using this model, the calculated point estimate of the peak force of infection is 0.282 (95% CI: 0.228- 0.353) at 5.0 years of age (95% CI: 4.3-6.0). Estimates of l by age groups may be obtained by integrating the above function over the appropriate interval, divided by the interval length. Discussion According to the Slovenian National Immunisation Pro- gramme guidelines, varicella vaccine is only recom- mended for individuals at the highest risk of complications, e.g. for immunocompromised subjects if their immune status permits the application of live, atte- nuated varicella vaccine. Also, the varicella vaccine is recommended for all those who reach adulthood with- out having been infected with varicella-zoster virus, par- ticularly for people with a professional risk for such infection (health care workers, kindergarten staff etc.). In practice, these broad recommendations are not fol- lowed and only a small proportion of people actually