E D I T O R I A L C O M M E N T A R Y Critical Contribution of Laboratories to Outbreak Response Support for Middle East Respiratory Syndrome Coronavirus Katrin C. Leitmeyer Microbiology Coordination Section, Office of the Chief Scientist, European Centre for Disease Prevention and Control, Stockholm, Sweden (See the Major Article by Drosten et al on pages 369–77.) Keywords. MERS; coronavirus; outbreak; nosocomial; Saudi Arabia. Middle East respiratory syndrome coro- navirus (MERS-CoV) was first detected in a patient living in the Kingdom of Saudi Arabia (KSA) in September 2012 [1]. As of 20 August 2014, 855 laboratory- confirmed cases of human infection have been reported to public health authorities worldwide, including 333 deaths. Most of the cases (97.5%) have occurred in the Middle East (KSA, United Arab Emirates, Qatar, Jordan, Oman, Kuwait, Egypt, Yemen, Lebanon, and Iran), with 723 cases alone reported from KSA. In all 21 cases that have been reported outside the Middle East (United Kingdom, Germany, France, Italy, Greece, the Netherlands, Tunisia, Algeria, Malaysia, Philippines, United States), the patients had lived in or traveled to the Middle East, or had contact with travelers returning from these areas [2]. Although the definite source of infec- tion and the exact routes of direct or indirect exposure remain unknown, the pattern of transmission pointed early to- ward an animal reservoir in the Middle East from which humans sporadically be- come infected through zoonotic trans- mission. Although the close phylogenetic relation of human MERS-CoV isolates ini- tially suggested bats as a putative natural reservoir [3], there is increasing evidence that dromedary camels serve as the primary source of MERS-CoV infection in humans [4]. MERS-CoV sequences have been iden- tified in dromedary camels’ nasal secretions [5, 6], but also in raw milk samples of in- fected animals [7]. There is serologic evi- dence for MERS-CoV infection in camels and calves from various regions in the Mid- dle East and beyond [8–10].Isolated viruses are matching to a high degree to the ge- nomes of the human MERS-CoV [11], and phenotypic characterization of viruses revealed that cell tropism and replication kinetics/competence of human and drome- dary MERS-CoV are similar [12]. Recently, virus replication and shedding was con- firmed in the upper respiratory tract of experimentally infected camels [13]. Besides the sporadic infections of hu- mans through zoonotic transmission, nos- ocomial transmissions contribute to the epidemiology. Person-to-person transmis- sion of MERS-CoV was shown to occur in healthcare settings with clusters among healthcare workers and patients in several countries [14–20]. Beginning in mid-March 2014, there was an explosive increase of MERS-CoV case notifications in KSA, with more cases reported in the month of April than during the 2 years following the be- ginning of the outbreak in March 2012 [21]. This raised important questions: Was this a real increase in cases or simply a surveillance artefact due to reporting of false positives or a change in surveillance practice with a more sensitive case detec- tion? Was this increase caused by an increased number of new introductions from the natural reservoir and/or did this increase indicate a breach in infection control measures that allowed increased nosocomial transmission? One of the most pressing questions, however, was whether there was evidence for a change in transmissibility of MERS-CoV en- abling a more efficient human-to-human transmission. On 26 April 2014, during the ongoing outbreak, an initial sequence analysis was communicated by Drosten and Corman indicating that the 3 MERS-CoV viruses recovered from cases in Jeddah exhibited a high degree of similarity to each other and a large number of known MERS- CoV sequences. This provided preliminary evidence that the virus had not undergone Received and accepted 1 October 2014; electronically pub- lished 16 October 2014. Correspondence: Katrin C. Leitmeyer, MD, MPH, Micro- biology Coordination Section, Office of the Chief Scientist, European Centre for Disease Prevention and Control (ECDC), Stockholm, Sweden (