Academic journal article Central European Journal of Public Health

Mathematical Modelling of Varicella Spread in Slovakia

Academic journal article Central European Journal of Public Health

Mathematical Modelling of Varicella Spread in Slovakia

Article excerpt

(ProQuest: ... denotes formulae omitted.)

INTRODUCTION

Varicella (chickenpox) as a highly contagious disease of children is caused by varicella zoster virus. The risk of complications depends on age and immune status. Source of infection is a person with manifest or inapparent infection. The disease is transmitted from person to person by direct contact and droplets (1, 2). Incubation period ranks from 14 to 21 days. The virus is highly contagious and can be spread 1-2 days before the rash appears until all blisters form crusts (usually 3-7 days) (3). Following chickenpox infection, the virus may remain latent in neural ganglia and after reactivation can be manifested as herpes zoster disease (shingles).

In Slovakia, 15,000-35,000 cases are reported annually (4, 5). Vaccination is the most effective method of prevention against varicella. Two types of varicella vaccine are currently available in the Slovak Republic. Tetravalent vaccine against measles, mumps, rubella and varicella (MMRV) can be provided either for children 15-18 months old, or in 11th year within mandatory regular vaccination. The monovalent vaccine can be used for vaccination of other people including adult persons. Vaccine is administered in two-dose schedule. In some countries (e.g. US, Germany), unlike Slovakia, it is included into mass vaccination. Vaccination against varicella has been available in the Slovak Republic only for a short time and only a small part of the population has been vaccinated. It is expected that significantly increased vaccination coverage could potentially cause decline in morbidity rate of the disease. The aim of this study is to predict the impact of vaccination on the occurrence of disease in different age groups using methods of mathematical epidemiology.

MATERIALS AND METHODS

Population Structure

Deterministic, age-stratified SIR (susceptible-infected-recovered) model (Fig. 1) was used for description of circulation of varicella in the Slovak Republic (6). Population with constant size of 5,000,000 inhabitants was due to comparability with data from EPIS (Epidemiological Information System) (5) divided into three child age groups (0-4 years, 5-9 years, 10-14 years), each of 300,000 children, and the rest of 4,100,000 inhabitants were considered adults (15+ years). The population was further divided into three groups according to the infectious status: S - susceptible, I - infected, R - resistant (sometimes called recovered). As susceptible is considered an individual who has not yet been infected and is at risk of infection; infected is an individual who can transmit pathogen to another individual; and resistant individual is a person, who has developed immunity after administration of vaccine or after overcoming the disease and is no longer susceptible to infection.

Transitions across the Groups

There are several types of transition across the groups in the model: ageing, natality, mortality, disease transmission, recovery, and vaccination. Mortality and birth rates have continuous character with the same value of 12[per thousand] annually (i.e. 60,000 births and deaths annually). The influx of new susceptible individuals was provided by births and the size of population was constant due to deaths (only in adult group). Every year (on the 365th day) one fifth of children (60,000 children) from every child age group passes to the next age group. This transition copies at least partially the mechanism of transfer of pupils and students across classes and schools.

Transitions due to disease transmission were described by transmission parameters, which are characterized by WAIFW matrix - Who Acquires Infection from Whom (6, 7). Recovery rate is an inverse proportion of duration of infectious period.

Vaccination of a group designated as 0 was provided continuously together with exchange of population - part of individuals entering the population remained susceptible, the other part has become immune. …

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