This is the prospective study that describes the epidemiology of HFRS in the ROK. This study determines the incidence, prevalence, seroprevalence, and subclinical infection rate in two high-risk groups, civilians in endemic areas and military personnel, during the epidemic months of the year. We identified HFRS cases through the active surveillance systems established in the endemic areas and the Korean troops. The incidence in the civilian group ranged from 2.1 to 6.6 per 100,000 person-months during the major epidemic period from 1996 to 1998. The estimated incidence rates rose to 3.9-7.6 per 100,000 person-months in Yeoncheon, when the follow-up loss cases were taken into account. Considering higher male to female ratio in the follow-up loss cases, the incidence might have been even higher since HFRS was more common among males, predominantly between 20 to 60 yr old (7
During the 3-yr study period, HFRS showed the peak incidence in 1998. Although the surveillance system was expected to find all HFRS cases within the study area, a significant numbers of cases were detected outside the active surveillance system. Severe cases showing typical clinical features of HFRS are likely to escape the surveillance system, because many of them are directly admitted to university hospitals by their family members who are well aware of the clinical manifestations of the disease. Accordingly, it was important to review the medical records of neighboring hospitals to detect additional HFRS cases. On the other hand, the active surveillance system is important in detecting the mild to moderate HFRS cases, which are more likely to go undetected without the serologic confirmation.
We estimated the prevalence of HFRS instead of incidence in Korean military personnel because there were limitations in access to the vaccination records. The annual prevalence from 1995 to 1998 was 40-64 per 100,000 military personnel, based on detection of the hospitalized cases. The hospital-based surveillance system for HFRS has been maintained in the AFCH since the 1980s; any symptomatic patients suspected to have HFRS are to be transferred and admitted to the AFCH. Review of the surveillance data from the ROK Army showed that the reported cases have distinctly decreased over the past years: from 166 to 73 cases (average, 130 cases) per year between 1981 and 1990, and from 59 to 23 cases per year (average, 37 cases) between 1991 and 1998. HFRS has been categorized as a reportable disease since 1997. Although incidence or prevalence studies have not been performed previously, it has been reported that more than 500 cases of HFRS were serologically confirmed annually among civilians and Korean military personnel in the 1980s (9
). There was some confusion or misunderstanding in the cited data for hospitalized civilian cases, rising to approximately 1,000 HFRS cases during the early 1990s; serologic tests were performed at several institutes with the samples from the same patient and each of the results were counted separately and then erroneously summed.
The reasons for the recent decrease in HFRS in the ROK have not been evaluated in depth. Possible related factors are improvement in the standard of living and housing in rural areas, ecological changes in rodent populations or other natural factors, and/or the effect of vaccination. The annual vaccination rates in civilians older than 20 yr old in the endemic areas were 1.4-5.5% during the study period. The vaccination rate in the military personnel from the 4 divisions of the Army in 1998 was much higher than civilians, ranging from 17.6 to 30%. Interestingly, the declining trend in the reported HFRS cases in the ROK Army in recent years is coincident with the start of vaccination in 1991. However, the effectiveness of HFRS vaccine should be evaluated in clinical trials.
There were mild fluctuations in HFRS incidence during the study period possibly related to the size of infected rodent population, climate, outdoor activities, and exploitation of fields and mountains. A seroepizootiologic study has reported annual fluctuating rodents' population in recent years (22
). The number of rodents trapped in 29 widely separated areas, including Apodemus agrarius
and Apodemus peninsulae
, was larger in 1997 and 1998, compared with those in 1996. The overall Hantavirus seroprevalence in trapped rodents was 13.1 to 13.6%.
Seasonal variation of HFRS is clearly observed in the military personnel, with the major peak occurring between October and December and the minor peak between May and July, which is consistent with previous reports. HFRS occurs throughout the year, but more than 85% of cases occurred during the major and minor epidemic periods (3
In our seroepidemiologic study, seroprevalence and subclinical infection rates among civilians and military personnel varied every year. The incidence of HFRS was high during the 1998 epidemic period and this seems to be due to low seroprevalence (7.0%) in the civilians before the epidemic. Relatively low incidence of HFRS during the 1997 epidemic can be related to relatively higher seroprevalence of 11%, which might have conferred immunity to HFRS. Song reported previously that relatively high subclinical infection rates after big outbreaks played a significant role in decreasing the incidence of HFRS (23
). As shown in our data, seroprevalence or subclinical infection rates in the civilian population were higher than those of the military personnel population. This might be due to perennial exposure of the civilian population to the high-risk environments compared to the military personnel who are exposed to the environment for, at most, three years during their enlistment. Interestingly, the highest subclinical infection rate of 15.7% and relatively high reported cases found in the military personnel during the minor epidemic period in 1995 could be due to possible mass exposure to a common source of infection. Seroprevalences of HFRS in the neighboring countries were comparable: 12% in rural China (24
) and 6.2% in Taiwan (25
According to the Korean CDMR (Communicable Disease Monthly Report), the reported cases of HFRS have slightly increased since 1999. 392 cases were reported in 2003 and, contrary to previous reports, many of them occurred in the southwestern area of the country (15
). However, the CDMR also included suspected cases in its analysis. Therefore, close observation is required in the following years.
A limitation of this study is that the analysis of the civilian population included data from only a 4 month period, from September to December. This limitation is difficult to overcome because long-term surveillance of rural civilian populations is technically challenging. Therefore, to get around this problem, we determined the exact incidence during the major epidemic periods when more than 75% of cases occur. Because of the limitation of this study design, we represented the incidences in civilians as person-months. However, the active surveillance system for the military personnel was in place year-round, making it possible to assess annual prevalence in military personnel.
Our data indicate that the incidence of HFRS in civilian populations in endemic areas varies every year, depending on the local seroprevalences and subclinical infection rates. Since rodents are major reservoirs for transmissions of Hantaan virus, the seroprevalence of the rodent population should be determined. Furthermore, the annual population cycle should also be determined to see how it is linked to annual incidence of HFRS. As for the military personnel, the prevalence remained generally the same throughout the study period with seasonal variations. Since the military personnel are a high risk group and the vaccination rate is about 30%, this population could be a good candidate for field trials. Our epidemiologic data on HFRS will be invaluable for establishing immunization strategies, immunization program policy, and vaccine efficacy studies.