Beyond the Routine Influenza Surveillance

Influenza surveillance systems have been developed to monitor antigenic changes in influenza viruses, guide the selection of annual influenza vaccine strains, and provide viral samples for vaccine production [1]. Subsequently, the need for epidemiological information has complemented virologic data collection. To adequately understand the burden and impact of influenza, morbidityand mortality-related data are essential. Such data can be obtained from sentinel healthcare providers at outpatient clinics and emergency departments, influenza hospitalization surveillance networks, and health statistics offices for mortality surveillance [2]. The accumulation of annual surveillance data allows rapid assessment of influenza burden, and it may detect the beginning of a pandemic. These data will provide timely information for public health-related decision-making. For the adequate measurement and comparison of disease burden, standardization of surveillance data is critical. Influenza-like illness (ILI) and severe acute respiratory illness (SARI) are the most commonly used proxy respiratory syndrome indexes of influenza surveillance. ILI is generally intended for use in outpatient settings and the SARI, in hospital settings that provide inpatient treatment. SARI is used with the aim of capturing both cases of pneumonia and aggravation of chronic diseases such as asthma, chronic obstructive lung disease, or congestive heart failure. Influenza epidemiology can differ depending on various factors including region, ethnicity, proportions of age groups, and influenza vaccination rate. In this issue of Infection & Chemotherapy, Kang et al. [3] reported the results of a multicenter prospective observational study on SARI and pneumonia in Korean adult patients who visited the emergency room with acute respiratory illness during the 2011-2014 flu seasons. Notably, the authors used the new category of “modified SARI.” The World Health Organization (WHO) case definitions for ILI and SARI include the requirement of measured fever of ≥38°C and presence of cough [4]. Fever is often masked in elderly or immunocompromised patients. Cough may not be a prominent symptom. Therefore, when SARI is used for surveillance, a substantial portion of influenza-related admissions are inevitably missed even if pneumonia or serious non-pneumonic complications are combined. Hence, the authors proposed the category of modified SARI, which included non-ILI laboratory confirmed influenza-related admissions as well as classical SARI defined using the classical WHO case definitions. Of 649 influenza-related admissions, 68 (10.5%) did not meet the ILI definition. Those cases included substantial numbers of cases of serious