Acute hepatitis A is the most common cause of acute viral infection, and the age group that is primarily affected is 20- to 30-year-old adults [1
]. In hepatitis A, which appears as an inapparent infection in cases where the original infection occurred during infancy, if the antibody retention rate decreases among infants, due to improvements in their living conditions and hygiene practice, there is a paradoxical tendency to have an increased risk, at initial presentation, of hepatitis A during adulthood [2
]. Assessing the epidemiological situation, the antibody cultivation rate was more than 50% until the 1980s. However, after the middle of the 1990s, the antibody cultivation rate decreased to less than 20%, and, in the past ten years, there has been an increasing tendency for the disease to spread among adults, who are easily exposed to the hepatitis A virus [3
]. When considering that the age group most prone to being infected by the hepatitis A virus is 20- to 30-year-old adults, who tend to have very active social lives, infection is not only a burden to society because of the direct medical expense, but also because of work absences or retirements that incur indirect social expenses [1
It is known that hepatitis A is related to the socioeconomic status and environmental hygiene of a country [8
]. First, in undeveloped regions, such as Africa, the Middle East, Central America, and some parts of Asia, that are categorized as highly dangerous [6
], most hepatitis A infections occur during infancy, and the antibody cultivation rate is above 90%. Second, developing countries and regions, such as Korea, Eastern Europe, and Russia, are considered intermediately dangerous [6
] because the antibody cultivation rate is above 50%, but among children ten years or younger, the rate is less than 20%. In regions with intermediate danger, due to the amelioration of the socioeconomic status and environmental hygiene, the rate of hepatitis A infection in children is not high, but when a group is infected, it has a tendency to increasingly occur as an apparent infection among adults aged over 20. Finally, developed countries and regions, such as Western Europe, Northern Europe, North America, and Japan, are categorized as not very dangerous [6
], but even within these regions and countries, in the areas where the socioeconomic status is low, the antibody cultivation rate is low; for those in groups that tend to have high infection rates, such as illicit drug users, men who have sex with men, travelers, and medical professionals, inoculation is strongly recommended [6
]. Likewise, the epidemiology of hepatitis A appears differently according to socioeconomic status and environmental hygiene, even within a single county [6
Outbreaks of hepatitis A have been increasing over the last ten years, alongside the improvement of socioeconomic conditions in Korea, and despite the fact that there are huge disparities in socioeconomic status among the regions, epidemiological studies targeting hepatitis A in Korea are rare [5
]. Many studies in Korea have explored the correlation between residential location and hepatitis A incidence, but they have not been able to find any correlation between the socioeconomic index, the hygiene standards, and the incidence, and the targeted research region was confined only to the capital of Korea [12
]. Thus, insufficient attention has been paid to a possible correlation between the incidence of hepatitis A and geographical location. Korea is a country with homogeneous social and housing conditions and a frequent exchange of travelers among regions, but hepatitis A incidence varies in the different regions. Therefore, grasping the primary factors that cause these huge regional differences in the hepatitis A incidence rate has public health implications [2
]. In cases where hepatitis A infection continues susceptible residents, who live in regions where the antibody retention rate is low, the public health implications are even greater, as the hepatitis A infection can spread and can become a major disease burden on the residents.
By using National Health Insurance data, which is a representative source of data on Koreans, and by targeting subjects infected with hepatitis A, the authors of this calculated the incidence of hepatitis A in each city, district, and borough (si, gun, and gu), and studied the relevance of socioeconomic status and environmental hygiene to hepatitis A outbreaks.
This study divided the Republic of Korea into 232 city, district, and borough sites, obtained the incidence rate of each region, and studied the correlation between the region's socioeconomic and environmental hygiene indexes and the incidence rate. The National Health Insurance data used in this study was appropriate for descriptive epidemiology, including time series study, as it is known to be a source that has a distribution similar to that of the actual patient distribution [21
This carefully examined other studies that showed a strong relationship between hepatitis A and regional characteristics, both inside and outside Korea. In Brazil, for example, the higher the education level, the water supply rate, and the solid trash pick-up rate by region, the lower the antibody cultivation rate, and this connection was statistically significant. Furthermore, in urbanizing regions, the state of public hygiene, such as the water supply rate, is a very strong risk factor for hepatitis A infection [11
]. In Taiwan, for example, the antibody cultivation rate was high in regions where the people drank dirty (non-tap) water [22
]. Moreover, among the 157 countries that have been studied, in studies where the authors were exploring the relationship between socioeconomic variables, including the water supply rate, the life expectancy, the gross domestic product, the education level, and the hepatitis A antibody cultivation rate, most of the variables were found to be significantly related. In addition, a strong negative correlation was found between the water supply rate and the antibody cultivation rate [8
]. According to research reported in Korea in 2007 [10
], the antibody cultivation rate was higher in rural areas than urban areas such as Seoul, especially among those under 40 years of age in Seoul; it was also significantly higher in the poorer regions than in the richer regions. Furthermore, hepatitis A is centered around Seoul and other big cities, and is concentrated in metropolitan areas [4
]. We analyzed the incidence rate by region in order to identify outbreaks of hepatitis A in Korea, from 2004 to 2008, to verify the hypothesis that hepatitis A is related to the socioeconomic and environmental hygiene levels of a region. We found that, in Korea, hepatitis A increased after 2004, and increased sharply in 2008, and that hepatitis A infection often occurs among young people in their 20s and 30s; these results agree with the existing study results [1
]. Therefore, Korea can be regarded as an intermediately dangerous region for hepatitis A infection [4
]. At the same time, except for some regions with disease epidemics, a high regional similarity exists in incidence every year. Because of environmental hygiene, the more a region's socioeconomic level, population density, and residence concentration (which results in more frequent contact among people) increased, the higher the incidence rate. In metropolitan areas, including Seoul and Incheon, there was a relatively high incidence rate. In addition, when looking at regional distribution in Korea, Korea's southwestern region had a higher infection rate than the other regions, which implies a regional epidemic of hepatitis A.
Hepatitis A, in a country like Korea with an intermediate danger of infection, the socioeconomic level and the antibody cultivation rate are inversely proportional [11
]. Furthermore, in a country like Korea, hepatitis A epidemics take place in regions with high socioeconomic status, where susceptible individuals are widely distributed. Korea was not equipped with water sewage systems until the 1980s, which led to the insufficient provision of drinking water, and because much of the farmland used feces as a fertilizer until the 1980s, most of the population was infected with inapparent hepatitis A in childhood via the fecal-oral route [2
]. After improving the hygiene conditions in the country, few adults who were in their infancy after the 1980s were exposed to the disease, and thus there are many adults who now susceptible to the disease. Therefore, after the 1980s, the way that people were exposed to hepatitis A infection changed. In other words, after the 1980s, it is assumed that the population group who spent their infancy in the city, where the socioeconomic level had greatly improved, probably had a lower chance of exposure to inapparent infection than the population group who spent their childhood in the countryside, where socioeconomic development was slow.
The results of the analysis in this study show that the lower the regional deprivation index, and the higher the educational level of the population, the greater the incidence rate of hepatitis A. This means that the higher the socioeconomic level of a region, the more susceptible the adults in their 20s and 30s will be to hepatitis A infection, as they have vigorous social lives, with frequent contact with people from a wide geographic distribution. Although we made an effort to adjust the age groups through direct standardization, the reason why the incidence rate shows differences according to the age structure is because each region has a different herd immunity level. In other words, regions with high socioeconomic levels have low herd immunity and show a high incidence rate. In terms of environmental hygiene conditions, the incidence rate increased in regions where there was a high population density, the residents could not access clean water, and household overcrowding was high. Thus, there was a tendency for the incidence rate to increase as the region developed conditions for people to have frequent contact. On the other hand, when looking at incidence rate results within the city of Seoul in 2008, there was a tendency for the incidence rate to increase where the deprivation index and socioeconomic levels were low. This is not the same as the results for the other regions of Korea, likely because in Seoul the socioeconomic level was low, the vaccination rate was low, and the hygiene conditions were not favorable. However, in this study, the authors could not confirm these results because there were not sufficient data to understand other hygiene conditions. Further study is needed in this regard.
In this study, the age group with the highest incidence rate varied according to the region: when the region was less developed, the incidence rate was at the highest level for those in their 20s, while in urbanized regions, the incidence rate was at the highest level for those in their 30s. The reason for this can be explained as follows: First, many of those in the 20s age cohort may actually reside in different regions than the ones stated on their social security cards. Many colleges in Korea are located in large cities where the deprivation rate is low. Thus, there may be a high ratio of college students in the 20s age cohort who live in metropolitan cities, but graduated from high schools in cities that are relatively less developed or have fallen behind. There is a tendency for these students not to transfer their residence registration address before they get a stable job; thus, if many of those in their 20s, who used to live in farming and fishing villages, reside in metropolitan cities, then in reality the incidence rate, according to the deprivation rate, could be different than what the data reflects.
Second, in regions with a low deprivation rate, namely regions in which there is a low population of those in their 20s, there is a possibility that an epidemic of hepatitis A has occurred. In regions where the population is low, even if there is an epidemic among a small number of men, when calculating the incidence rate, there is a possibility that the incidence rate will increase exponentially.
Third, depending on the socioeconomic status and environmental hygiene level of a region, there are different age groups that contract hepatitis A and acquire the antibody. According to a study performed in Brazil, in which the danger level is intermediate [11
], wherever the socioeconomic status varied, the antibody cultivation rate differed according to age group. In the most backward region, the Northeastern region, the hepatitis A antibody cultivation rate before the age of five was 31.5%, and in the federal region, where it was most developed, the cultivation rate was only 20%. However, when the population reaches 19 years old, both regions have a similar antibody cultivation rate of 70%, and this shows that the more developed a region is, the higher the age group that acquires the hepatitis A antibody. Therefore, the age variation phenomenon exists according to the socioeconomic status of a region.
The results of this study are comparable to earlier studies. As in this study, previous research has shown that individuals living in regions with a high socioeconomic status have a lower possibility of inapparent infection during infanthood than those in regions with a low socioeconomic status. Nevertheless, a low education level and a low rate of water supply are understood as risk factors for hepatitis A infection [11
]. In addition, the results of this study indicate that the contraction rate of hepatitis A increases with population density, which matches previous research results [25
There are a couple of limitations to this First, the incidence rate was calculated using only patients whose medical fees for hepatitis A infection were covered by the National Health Insurance Corporation, and thus it was impossible to understand the scale of the inapparent infection of hepatitis A. In reality, hepatitis A is easily ignored during infancy because it appears as an inapparent infection, and thus it is difficult to determine the number of childhood cases [23
]. Second, when calculating the incidence rate, there is a possibility that the incidence rate could be greatly underestimated because the denominator includes people who already possess the antibody. Furthermore, in farming and fishing villages where the population was heavily weighted toward older people, such a tendency was intensified. Thus, with respect to the antibody cultivation rate for every region, underestimation of the incidence rate should be considered. Third, although there are different probabilities for being infected with hepatitis A according to an individual's education level and occupation [19
], and although some previous studies have implemented multi-level analysis of individual and regional socioeconomic levels [11
], this study failed to analyze the incidence aspect at an individual level due to the limited availability of data.
Despite its limitations, this study is significant for several reasons. First, this study has undertaken a process of targeting all citizens registered in the residence registration statistics, and this likely produced representative data that can reflect various aspects of hepatitis A incidence in Korea. Second, previous studies have targeted certain regions of Korea [12
], but this study targeted the entire country. Third, this study analyzed not only the regional incidence rate itself, but also the relationship between the region's socioeconomic level and the incidence rate. In addition, this study is significant in the way that it explores the reasons for the regional differences in hepatitis A incidence. This study hows that there can be changes in the hepatitis A incidence rate related to the socioeconomic status and environmental hygiene of Korean regions. It is a very important public eubiotic problem that, in epidemics of hepatitis A in a country with intermediate danger such as Korea, the incidence rate is higher in regions with a low antibody cultivation rate.
There are different trends in Korea according to regional socioeconomic status. When a region is socioeconomically backward, it is close to the epidemiological concept of a highly dangerous region, where there is much inapparent infection at an early age; on the other hand, the more socioeconomically developed a region is, the older the age of the infection class. In the current epidemic period, many susceptible people from socioeconomically developed regions are infected. In order to overcome this and become a low danger country, not only is it necessary to have well-balanced development, but also aggressive prevention, which should be directed at young people who are susceptible and live in regions where there is a high danger of hepatitis A infection.
The primary methods for preventing hepatitis A infection are the improvement of public health conditions (in order to stop the spread of the hepatitis A virus), thorough individual hygiene maintenance, and hepatitis A vaccination [2
]. Presently, hepatitis A vaccination is not a national mandatory vaccination in Korea, but rather an optional vaccination; adults over 40s are not recommended to receive vaccination. The government of strongly recommends the vaccination of young children up to 16 years old who do not have the hepatitis A virus antibody, and those in their 20s and 30s with hepatitis B, hepatitis C, or orchronic epilepsy, as well as those who plan an extended stay in a hepatitis A-infected area, laboratory practitioners, and those who have come into contact with a hepatitis A-infected person within the previous two weeks [27
However, as one can see from the present study, the epidemiological aspects of hepatitis A vary by region. Once an epidemic has begun, vaccination should be provided not only to those in highly endangered groups, but also to the general public in highly dangerous areas with low antibody cultivation rates, and the vaccination should be performed strategically.
Presently, the teenage population and those in their 20s have the lowest hepatitis A cultivation rate in Korea [7
]. In the case of developed countries, considering the fact that it has taken ten years for the hepatitis A virus to be transferred to an intermediate epidemic trend region to low epidemic region [2
], it can be predicted that there is a possibility that the number of adult and youth hepatitis A-infected patients could grow rapidly grow. Based on these results, strategic vaccination is highly necessary, targeting youths and adults with high susceptibility. In reality, when looking at Israel, a country that is classified as an intermediate-danger region, after they performed a routine vaccination, where children between the ages of 1 and 2 were vaccinated, the number of patients with hepatitis A dramatically decreased [7
]. However, considering the existing circumstances, in which a vaccination campaign targeting the whole nation is difficult due to high cost, it is very important to develop various strategies for vaccinating susceptible adults [2
This study shows that differences in socioeconomic status can be a risk factor for hepatitis A infection. In case the hepatitis A infection epidemic trend continues, public health measures, such as strategic vaccination, ensuring a safe supply of drinking water, and being responsible for the clean handling of food ingredients, can be utilized.