INTRODUCTION
Middle East Respiratory Syndrome (MERS) is a severe respiratory infection caused by a novel beta coronavirus (MERS-CoV) [1-3]. The symptoms of MERS include fever, chills, cough, shortness of breath, gastrointestinal symptoms, expectoration, wheezing, chest pain, hemoptysis, sore throat, headache, myalgia, abdominal pain, vomiting, and diarrhea; it can also cause death in severe cases [3-6].
The causative pathogen of MERS is transmitted via 4 modes: animal-to-human, intra-familial, healthcare-associated, and travel-related [7,8]. The 186 cases that occurred in South Korea (hereafter Korea) were predominantly caused by healthcare-associated transmission [7,9-11], followed by intra-familial transmission.
According to data reported to the World Health Organization, the rates of asymptomatic or mild infection were 44 of 398 (28.60%) in Saudi Arabia, the United Arab Emirates, and the Islamic Republic of Iran between April and June 2014, and 32 of 113 (28.31%) in Saudi Arabia in June 2014 [12,13]. However, Oboho et al. [14] reported that 78.79% (26 of 33) of initially reported asymptomatic patients had at least 1 symptom. In Korea, among the 186 confirmed cases, 3 asymptomatic cases were detected among healthcare workers via screening tests (1.61%) [15]. In serologic studies using indirect immunofluorescence tests for healthcare workers who were at MERS-affected hospitals, 2 of 457 (0.44%) had positive results [16]. However, no report has been published regarding the asymptomatic infection rate among non-healthcare workers in Korea. There is a considerable chance of human-to-human transmission, as well as direct infection via the dromedary camel [17-19]. Therefore, it is necessary to identify the rate of asymptomatic MERS infections in healthcare workers and non-healthcare workers.