Varicella is the primary disease caused by the varicella-zoster virus. It is a common and highly contagious disease and has a significant health impact on children. Although the clinical course of varicella is usually mild and self-limiting, varicella does cause complications and mortality resulting in financial expense [1–3]. Chickenpox or varicella now has been considered one of the most common vaccine-preventable diseases in many countries [4–7].

In Taiwan, some areas including Taipei City, Taichung City and Taichung County gave the public free varicella vaccinations for 1-2 year-old children before 2004. Taipei City started the free varicella vaccination from 1998 and Taichung City and Taichung County from 1999. Other areas did not provide free varicella vaccination, but people could receive varicella vaccination at their own expense before 2004. Since 2004, free mass varicella immunization has been given to all one year-old children throughout Taiwan.

Since the implementation of National Health Insurance (NHI) in 1995, there has been a healthcare database in Taiwan. Taiwan has a population of 22.9 million people and the land area of 36188 Km². Its population density is 633/Km². Taiwan's NHI covered most of the health care costs for 98% of its population in 2006 [8]; the remaining 2% of it population are living in the foreign countries or in families with monthly household incomes less than 1000 US dollars. Taiwan's NHI database includes health care data collected from over 95% of the hospitals in Taiwan for more than 96% of the population receiving health care. We evaluated the disease burden and epidemiological characteristics of varicella by using the NHI database to find out the age-specific incidences, seasonal distribution and varicella-related hospitalization from 2000 to 2008-- the period covering pre- and post-public free vaccination-- so we could compare the epidemiological characteristics of varicella before and after the mass immunization.

Nationwide varicella immunization has resulted in a marked reduction of varicella incidence and varicella-related hospitalization in children younger than 6 years of age in Taiwan. The impact of mass varicella immunization has so far made it possible for most preschool children to be free of varicella and varicella-related hospitalization in Taiwan. However, children beyond 7 years of age had similar annual incidence before and after the mass immunization.

As Figure 3 shows the age distribution before the mass immunization in 2000, the peak age of varicella was 3 to 6 years, which was also the age group of kindergarteners. Kindergarteners might have more exposure to varicella and also their hand hygiene and sanitation levels are generally not as high as adults' or older school children's, who may in turn lead to more varicella cases in kindergarteners without mass immunization (Figure 3). However, after the mass immunization, we observed more varicella cases in the school children than those in 3 to 4-year-old kindergarteners (Figure 4), who were supposed to have received the free varicella vaccine at 1 year of age. Therefore, mass immunization has changed the age distribution of varicella cases significantly and the peak age has subsequently moved to older children without previous immunization. Similar results were also reported by Lian et al who found that the greatest decrease in varicella incidence occurred in children aged below 6 and the incidence of varicella shifted to older age groups after implementation of the one-dose varicella vaccination policy [11]. Lian et al also found that the incidence in early launch areas was significantly lower than that in the other areas in Taiwan [11]. Our study additionally reported the seasonality, varicella-related hospitalization and complications from 2000 to 2008 in Taiwan. Moreover, our data in this study is nationwide rather than the selected sample size like that of Lian et al's study.

We found that the annual incidence of children beyond 7 years of age was similar before and after mass immunization. It's because most of the school children were not allowed to receive the free varicella vaccine. Thus, the varicella incidence of the school children did not change significantly after the mass immunization in this study. We suppose that the annual incidence of school children may decrease several years later if the immunized children grow up to be 6 to 12 years old. However, we have to follow up on the impact of mass immunization on school children to prove the above assumption several years later.

The infants had the highest annual incidence of varicella-related hospitalization both before and after the mass immunization (Figure 6). Infants cannot receive the varicella vaccine and they have a higher chance of severe varicella and more complications [12], so their varicella-related hospitalization was significantly higher than the other children.

There were 12 cases of fatality, of which 5 of them had underlying diseases. Children with underlying diseases such as acute leukemia are more susceptible to getting severe varicella with complications and have a significantly higher mortality rate [12]. Therefore, the varicella vaccine may be administered to children with acute lymphoblastic leukemia who are in remission, or, high-titer anti-varicella-zoster virus immune globulin as post-exposure prophylaxis is recommended for immunocompromised children and newborns that are exposed maternally to varicella [12].

Currently, the varicella vaccine program consists of a single-dose vaccination in Taiwan, as it is in Canada, Korea and Australia [13]. However, both varicella outbreaks and breakthrough infections have occurred after the single-dose varicella vaccine program was initiated [14]. These might be related to a loss of immunity or to primary vaccine failure. Although the reports are limited, varicella vaccine effectiveness may be higher when two doses are administered than when a single-dose is administered. In 2006, the United States changed its immunization policy by stipulating that 2 doses be administered, once at 1 year of age and another administered between the ages of 3 and 4 years old [15]. In Taiwan, boosters in 4-6 year-old children to reduce the breakthrough infection or amend the primary vaccine failure may be considered if further epidemiology reveals frequent outbreaks or breakthrough infections in kindergarteners.

The major limitation of this study is that we could not verify the diagnoses because we could just analyze the NHI database rather than detailed medical charts. However, at the very least the nationwide database can let us outline the comprehensive picture of varicella epidemiology in Taiwan and sketch the impact of nationwide immunization.

In conclusion, nationwide varicella immunization significantly decreased the varicella annual incidence and varicella-related hospitalization in preschool children in Taiwan. The impact of the varicella vaccine on schoolchildren may be followed up on several years later.