Sočan et al. BMC Public Health 2010, 10:360 http://www.biomedcentral.com/1471-2458/10/360 varicella as a serious disease. A concern has been raised that widespread childhood varicella vaccination will shift the peak incidence of the disease from children to young adults and result in higher numbers of severe cases [20]. Mathematical modelling suggested that rou- tine varicella vaccination might generate an upward trend in herpes zoster incidence at least in the short to medium term [22,23]. Implementation of the universal varicella vaccination programme in the USA was followed by a rapid decline in incidence of varicella among immunized children, associated with a cohort effect in the non-immunized population. The initially recommended one-dose sche- dule provided only limited protection as a result of primary and/or secondary failures, i.e., waning vaccine- derived immunity. In 2007, the ACIP (Advisory Committee on Immunization Practices) advised that the one-dose schedule be replaced by a two-dose schedule in the USA. The interval between the first and the sec- ond dose may be short (but should be at least one month), standard (3 to 7 years apart), or even longer, depending on varicella epidemiology [24]. Thus, the decision concerning an optimal vaccination schedule depends greatly on the local epidemiological situation. Slovenia is among the countries where a routine vari- cella vaccination programme has not yet been imple- mented. The present study was designed to investigate the Slovenian epidemiological situation in order to aid planning of the optimal varicella vaccination strategy in Slovenia. For this purpose, a cross-sectional survey of the seroprevalence of VZV antibodies was performed and the collected data were used to assess the force of VZV infection. The force of infection is a central parameter in the assessment of an infection within a population. The force of infection is the rate at which susceptible indivi- duals become infected by an infectious disease, whilst taking susceptibility into account. The measure can be used to compare the rate of transmission between differ- ent groups of the population for the same infectious dis- ease. It depends on a variety of age-dependent factors, is thus itself age-dependent and, therefore, provides the key information regarding childhood vaccination schedule. Methods Seroprevalence survey For the cross-sectional, age-stratified seroprevalence sur- vey, a total of 3,689 sera from individuals aged 0-97 years were tested for VZV IgG antibodies. Serum sam- ples were collected at the Clinical Institute of Clinical Chemistry and Biochemistry Ljubljana in 2006 using residues from specimens taken for routine diagnostic tests. Page 2 of 6 The overall age structure of the total sample was chosen according to the ESEN (European Seroepidemio- logical Network) specifications: about 100 samples were obtained per each of the 20 age groups 0-19 years and about 200 samples per each of the 7 age groups ≥20 years (20-24, 25-29, 30-34, 35-39, 40-49, 50-59, ≥60) with approximately equal number of samples by gender in each of the groups [25]. The anonymised samples, marked only with the date of sampling, age and sex of the patient, were sent to the Laboratory for Virology at the Institute of Public Health where varicella antibody assays were performed. The sera were stored at -20°C before being tested with a commercial enzyme-linked immunoassay (ELISA; Enzygnost Anti-VZV Virus/IgG, Dade Behring, Mar- burg, Germany) that was used for the determination of specific IgG antibodies against VZV. The assay was run in accordance with manufacturer’s instructions. Optical density measurements were performed and results were calculated using an automated system (Dade Behring BEP III System, Dade Behring, Marburg, Germany). All tests were evaluated using adequate positive and nega- tive reference controls according to the manufacturer’s instructions. Results were classified as positive, negative or equivocal. Specimens with equivocal results were retested using the same test kit and those few that were equivocal again were classified as negative [26]. The protocol for the seroprevalence survey was approved by the National Medical Ethics Committee of the Republic of Slovenia. 2.2. Estimation of the force of infection Two different approaches were employed to estimate the force of VZV infection based on our data. The first approach was based on a semiparametric model (piece- wise constant force of infection) as described by Mos- song [27]; this approach also provided the possibility of direct comparison with some recent results reported for other European countries [16]. The second approach was based on the use of a parametric (catalytic) model as reported by Farrington and based on the work of Griffiths [28,29]. Such a two-tier combined evaluation seemed appropriate, as both semiparametric and para- metric methods have limitations [30]. A detailed description of the modelling is given in Additional file 1. All modelling and data analyses were performed using the R statistical software package version 2.9 [31]. When estimating the force of infection based on sero- logical data, some basic assumptions are made and certain methodological issues have to be considered. These are covered elsewhere, but two issues should be noted here [29,32,33]. Firstly, inherited maternal antibodies, which yield a high proportion of seropositive infants, lead to low