Super-spreading events of MERS-CoV infection.

Middle East respiratory syndrome coronavirus (MERS-CoV) was first identified in September, 2012, from a 68-year-old man who had died of severe pneumonia and multiorgan failure in Saudi Arabia in June, 2012. Since then, MERS-CoV infection has spread to 27 countries, including South Korea, where 186 cases had been confirmed within 2 months following the return of a Korean businessman (Patient 1) who had visited four countries in the Middle East between April 18 and May 3, 2015. This major outbreak in South Korea is characterised by five super-spreading events in hospital settings, of which two were related—one at Pyeongtaek St Mary's Hospital (Pyeongtaek; by Patient 1) and one at Samsung Medical Center (Seoul; by Patient 14).

During Patient 1's stay at Pyeongtaek St Mary's Hospital on May 15–17, 2015, he had infected 36 patients, including Patient 14, who was staying on the same floor. Both Patient 1 and Patient 14 had sought medical attention at different health-care facilities before being treated at the Samsung Medical Center on May 17–20 (Patient 1) and May 27–29 (Patient 14), around day 7 of their illness when they were highly infectious. However, it was Patient 14 who had led to the major nosocomial outbreak at the Samsung Medical Center. Patient 1 had initially presented to the emergency room on May 17, 2015, when the hospital was full; he was admitted and isolated immediately on May 18, 2015, after his travel history to the Middle East was ascertained by a medical officer, without causing any nosocomial outbreak.

In The Lancet, Sun Young Cho and colleagues report results from their comprehensive retrospective investigation of the MERS-CoV super-spreading event at the Samsung Medical Center emergency room, including a review of closed-circuit security video footage and electronic medical records. This nosocomial outbreak was most intriguing, with 82 people (33 patients, eight health-care workers, and 41 visitors) being infected following exposure to Patient 14 on May 27–29 in the emergency room. Patients staying in the same zone as Patient 14 had the highest attack rate (20% [23 of 117 patients]), compared with 5% (three of 58) in those with brief exposure to Patient 14 at the registration area or the radiology suite of the emergency room, and 1% (four of 500) in other patients who stayed in different zones. The median incubation period of patients who stayed in the same zone as Patient 14 was shorter than that in patients who stayed in different zones (5 days [IQR 4–8] vs 11 days [6-12]; p<0·0001). No cases were documented in patients and visitors who had visited the emergency room on May 29 and were exposed only to potentially contaminated environment without direct contact with Patient 14. The data suggest that the location (and hence the timing) of exposure to Patient 14 was an important factor in determining the attack rate and incubation period.

Several other predisposing factors to this super-spreading event included failure to implement strict isolation of patients and quarantine of contacts at the first outbreak hospital (Pyeongtaek St Mary's Hospital),3, 4 poor communication and knowledge of patient movement between hospitals, overcrowding in the emergency room, inadequate ventilation with only three air changes per h, and limited availability of isolation rooms in the emergency room. At least six air changes per h in existing hospital facilities are needed to reduce room contamination in the management of acute respiratory infections, whereas 12 air changes per h are recommended for new or renovated facilities, especially when managing patients receiving mechanical ventilation and during aerosol-generating procedures.

Failure in infection control and prevention in health-care facilities has resulted in large numbers of secondary cases of MERS-CoV infection involving health-care workers, existing patients, and visitors in Saudi Arabia7, 8 and several other countries in the past few years.5, 9, 10 Common risk factors include exposure to contaminated and overcrowded health-care facilities, poor compliance with appropriate personal protection equipment when assessing patients with febrile respiratory illness, application of potential aerosol-generating procedures (eg, resuscitation, continuous positive airway pressure, nebulised drugs), and lack of proper isolation room facilities.5, 7, 8, 9, 10 The customs of patients seeking care at different health-care facilities (so-called doctor shopping), as in the cases of Patients 1 and 14, and having friends and family members to stay with patients as caregivers at already overcrowded health-care facilities are unique factors in South Korea.

Although no aerosol-generating procedures were performed (with the exception of Patient 14 receiving supplemental oxygen at 2–5 L per min during his stay at the emergency room), the role of such procedures, environmental contamination, and asymptomatic carriers in disease transmission would require further investigation in future major nosocomial outbreaks of MERS-CoV infection. Good compliance with appropriate personal protection equipment by health-care workers when managing patients with suspected and confirmed MERS-CoV infection, early diagnosis, prompt isolation of infected patients, and improvement of ventilation in health-care facilities are important measures to prevent nosocomial outbreaks.