Background: Vaccination against hemorrhagic fever with renal syndrome (HFRS) has been applied successfully for more than 20 years in China, and since 2008, the government has implemented the Expanded Program of Immunization (EPI) in regions with high incidence. In this study, we analyzed the EPI-related changes in age distribution in reported cases of HFRS and proposed new recommendations for prevention and control of the disease. Methods: Data relating to incidence of HFRS, geographical location and age distribution were collected through the China Information System for Disease Control and Prevention (CISDCP) from 2005 to 2010. Excel and SPSS 18.0 software, χ2 tests and descriptive methodology were used for analysis of the data. Results: A total 75 434 HFRS cases were reported in 28 provinces in China between 2005 and 2010. The majority of HFRS cases occurred in adults aged 30 to 55 and this group accounted for 68.3% of the total. With the implementation of the immunization program, HFRS age distribution has clearly changed in recent years. The proportion of HFRS cases among individuals targeted by EPI (16–60 years of age) decreased from 86.9% in 2005 to 81.9% in 2010. However, the proportion of cases among the non-vaccinated group aged 60 had increased from 13.1% in 2005 to 18.1% in 2010. Notably, in the >60 age group the proportion rose from 8.8% in 2005 to 14.7% in 2010. These differences were statistically significant. Conclusion: HFRS vaccination has played an important role in HFRS control and prevention in China. However, since the proportion of HFRS cases over 60 years old has increased significantly since EPI was implemented, it is recommended that the age limit for vaccination be reconsidered. This finding may have practical implications for more effective HFRS vaccination in the future.

Severe fever with thrombocytopenia syndrome (SFTS) is a severe viral disease caused by SFTSV. It is important to estimate the rate of missed SFTS diagnosis and to further understand the actual incidence in high endemic areas in China. This study was conducted in two high SFTS endemic provinces in 2015. Patients hospitalized in 2014 or within 1 year before investigation were selected after considering their clinical manifestations, specifically, fever, platelet, and white blood cell. During retrospective investigation, sera were collected to detect SFTSV antibodies to assess SFTSV infection. To further understand SFTSV infection, acute phase sera were detected; SFTSV infection rate among a healthy population was also investigated to determine the basic infection level. In total, 246 hospitalized cases were included, including 83 cases (33.7%) with fever, thrombocytopenia and leukopenia, 38 cases (15.4%) with fever and thrombocytopenia but without leukopenia, and 125 cases (50.8%) without fever but with thrombocytopenia and leukopenia. In total, 13 patients (5.3%) were SFTSV IgM antibody-positive, 48 (19.5%) were IgG-positive. Of the 13 IgM-positive cases, 11 (84.6%) were IgG-positive (9 with titres ≥1:400). Seropositive rates of antibodies were high (8.4% for IgM and 30.1% for IgG) in patients with fever, thrombocytopenia and leukopenia. Furthermore, among IgG-positive cases in this group, 76% (19/25) of patients’ IgG antibody titres were ≥1:400. Additionally, 28 of 246 cases were initially diagnosed with suspected SFTS and were then excluded, and 218 patients were never diagnosed with SFTS; the seropositive rates of IgM and IgG in these two groups were 25% and 67.9% and 2.8% and 13.3%, respectively. These rates were 64.3% and 21.4% in 14 sera collected during acute phase of the 28 cases mentioned above. Seropositive rate of SFTSV IgG was only 1.3% in the patient-matched healthy group, and no IgM antibody was detected. A preliminary estimate of 8.3% of SFTS cases were missed in SFTS high endemic provinces. The actual SFTS incidence was underestimated. Effective measures such as adding a new SFTS case category - “SFTS clinical diagnosis cases” or using serological detection methods during acute phase should be considered to avoid missed diagnoses.