Hantavirus infection causes well-known clinical syndromes including hemorrhagic fever with renal syndrome (HFRS) and hantavirus cardiopulmonary syndrome (HCPS). During the Korean War, thousands of hemorrhagic fever outbreaks were reported in the early 1950s [1]. In 1976, the etiologic agent, Hantaan virus (HTNV), was isolated in lung tissues from the striped field mouse, Apodemus agrarius, by specific immunofluorescent reactions with HFRS patient serum [2]. In the 1980s, more hantavirus species were isolated worldwide from HFRS and HCPS outbreaks. Hantaviruses produce a chronic infection in rodent hosts. Rodent reservoirs then transmit hantavirus to humans via aerosolized rodent excreta [3]. HFRS is a clinical syndrome associated with significant acute renal impairment and mortality. Approximately one-third of HFRS cases include stage 3 acute kidney injury (AKI) according to Kidney Disease: Improving Global Outcomes (KDIGO), with 5% to 10% overall mortality [4].

Globally, most recent HFRS cases have been reported in East Asia, followed by Russia and Europe. Infections by HTNV, Seoul, Dobrava, and Pluumala viruses have been reported in these areas. In South Korea, HTNV has been the most common causative pathogen of HFRS [5]. Hantavirus is a zoonotic infection hosted by small mammals such as rodents. Humans are generally considered dead-end hosts of hantaviruses as no human-to-human transmission of hantavirus has been reported except for a few reports of HCPS with the Andes virus [6]. Therefore, severity of renal complications and mortality of HFRS patients have been emphasized rather than transmission between humans.

Because no specific therapy has been developed, several prevention strategies have been established including avoiding rodent habitats and inhalation of contaminated dust and vaccine-based immunization. In 1988, the first inactivated HTNV vaccine (iHV), Hantavax^® (Korea Green Cross, Seoul, Korea), was developed from cultured brain cells of suckling mice infected with HTNV [7]. High-risk individuals who were unable to avoid rodent infested areas were provided with iHV in the Republic of Korea (ROK), particularly members of the ROK Army and rural communities [8]. The ROK Army provided additional preventive measures including avoiding training sites in suspected areas of increased HFRS risk, eliminating rodents, and cutting down bushes or tall weeds in adjacent areas [9]. Following the implementation of these efforts, the number of HFRS cases in the ROK Army has declined since the 2000s; however, dozens of HFRS cases still occur annually among the ROK Army population [10]. Furthermore, clinical studies to prove the protective efficacy of the iHV remain inconclusive.

The present study aimed to assess iHV efficacy against disease progression in severe HFRS cases in the ROK Army. To address iHV efficacy, a retrospective study was designed utilizing pooled surveillance data of HFRS cases and comprehensive clinical records from ROK Army hospitals.

A total of 192 HFRS subjects were enrolled in the program based on the study protocol. The reasons for exclusion included no available vaccination record (n = 12), absence of clinical data for severity estimation (n = 12), false-positive rapid immunochromatography assay findings due to prior vaccination (n = 24), and no evidence of renal involvement (n = 11). Among the 133 subjects, 23 had received at least one vaccination. According to the definition of valid vaccination, 18 patients were included in the valid vaccination group, while the other five patients were included in the invalid group due to the time elapsed since the last vaccination (Fig. 1).

Clinical parameters and event rates were compared within the non-vaccination (n = 110) and valid vaccination (n = 18) groups. The invalid vaccination group (n = 5) was excluded from analysis due to insufficient numbers. All patients were male with a mean age of 21.6 years (interquartile range, 20 to 22 years). No significant differences in age, baseline body weight, body mass index, and year at time of occurrence were observed. The patients’ military rank at admission was significantly lower in the non-vaccination group compared to the vaccination group (Table 1).

The maximum weight gains from baseline were 8.0% and 8.2% in the non-vaccination and valid vaccination group (P = 0.613), respectively. Clinical events such as pulmonary edema or intensive care unit (ICU) admission, mechanical ventilator use, renal replacement therapy, and inotropic agent administration did not differ significantly between the two groups. The clinical outcome expressed as KDIGO-AKI stage did not differ statistically between the two groups (P = 0.285). The proportion of patients with stage 3 AKI was 54.5% in the non-vaccination group and 33.3% in the vaccination group (P = 0.128). The duration of ICU stay among patients who received intensive care was 5.6 days in the non-vaccination group and 5.2 days in the valid vaccination group (P = 0.626). The mean length of hospitalization was 16.7 days in the non-vaccination group and 14.8 days in the vaccination group (P = 0.189) (Table 2).

The VE_p of the iHV was 58.1% as defined by stage 3 AKI (95% CI, −31.3% to 88.0%) and 57.3% as defined by dialysis (95% CI, −102.1% to 95.5%). Neither VE_p definition value showed statistical significance (Table 3).

While the peak serum aminotransferase level was significantly higher in the non-vaccination group (95% CI, 13.0 to 118.7), the platelet and white blood cell counts, serum uric acid and creatinine levels, and urine protein/ creatinine ratio did not differ significantly between the groups (Table 4).

The effectiveness of the iHV on disease progression in HFRS patients was investigated. The VE_p values were 58.1% for stage 3 AKI and 57.3% for patients with AKI who received renal replacement therapy. The overall prevalence of severe cases was 52.6% (70 of 133 HFRS patients) when defined as stage 3 AKI and 20.3% when defined as receiving renal replacement therapy when defined as stage 3 AKI (27 of 133 patients). Only two cases of mortality occurred during the study period, and neither patient was vaccinated. Therefore, the iHV may have moderated HFRS disease severity in the military population.

Another studies have reported the efficacy of the iHV on the severity of HFRS [12]. Two studies included a control group within an ROK Army battalion in an HFRS endemic area [13,14]. The vaccine effectiveness against disease onset was calculated by comparison of HFRS and non-HFRS patients in the same hospital and showed a statistically significant effectiveness of the iHV to reduce disease prevalence by approximately 60% [14]. In contrast, our study assessed the vaccine effectiveness based on HFRS disease progression, assessing HFRS severity among iHV vaccination and non-vaccination groups.

This study is also characterized by vaccination status of study subjects. Only two of 23 vaccinated patients completed the three dose protocol of the iHV. Several studies evaluating iHV immunogenicity have consistently reported high positive seroconversion rate confirmed by immunofluorescent antibody assay after vaccination. The reported seroconversion rate at one month after the first dose ranged from 50% to 80%, which increases to 80–100% after the second dose. The seroconversion rate then dropped at 12 months in approximately 30% to 40% of subjects. An iHV booster dose at 13 months successfully raises the seroconversion rate by 90% to 100% [7,8,11]. In this study, 13 of 23 patients had received only one iHV dose at the time of disease occurrence. Another HFRS study in the ROK Army showed vaccination status in HFRS patients as 37% for one dose, 35% for two doses, and 28% for three doses [13]. While the complexity of vaccination status among patients was suited for practical application, further study of dose-related efficacy of iHV is required.

This study has several limitations. Due to its retrospective and observational study design, mild cases were included in the preliminary data of the vaccination group. However, preliminary registration and medical history record review identified that more than half of the vaccinated patients were misdiagnosed with HFRS due to false-positive rapid immunochromatography assay. We confirmed the status of the valid-vaccinated HFRS cases by excluding all uncertain cases by clinical misdiagnosis and false-positive serological test findings. In comparison, fewer HFRS unvaccinated patients were excluded; less than one-tenth of the unvaccinated subjects did not meet the clinical criteria of HFRS. The selective bias in these groups may have affected the true vaccination effect on disease progression. If the less severe cases were more often excluded in the vaccination group compared to the non-vaccination group, the VE_p may have been underestimated. Second, the value of vaccination effectiveness on disease progression failed to show statistical significance. This result was partially attributed to insufficient numbers of validly vaccinated subjects. Furthermore, the unequal numbers of subjects between the two groups resulted in nonparametric analysis, lowering the statistical test values. Lastly, differences in the study population characteristics prevented generalization of the results to the entire population. All of the patients in this study were men, the vast majority of which were in their twenties. Since sex differences in the incidence and mortality of HFRS have been reported, further research is needed to determine the effectiveness of the iHV program in the general population [15,16].

In conclusion, this study described the clinical presentations and disease courses of patients with HFRS in South Korean military areas and compared two groups according to vaccination status. Vaccination appeared to reduce HFRS progression. The prevalence of severe renal disease, defined as stage 3 AKI or receiving renal replacement therapy, among iHV vaccinated patients was approximately half that of unvaccinated patients. This result implies a positive preventive effect of the iHV against severe HFRS outcomes. The effectiveness of the iHV was suspected to affect not only HFRS incidence, but also HFRS progression. However, because the test value of the analysis in this study did not reach statistical significance, further studies with larger numbers of subjects are required.