Severe acute respiratory syndrome — Singapore, 2003

Morbidity and Mortality Weekly Report, May 9, 2003

The Singapore Ministry of Health (MOH), with assistance from the World Health Organization (WHO), has been investigating an outbreak of severe acute respiratory syndrome (SARS). This is a novel condition caused by the SARS-associated coronavirus (SARS-CoV) and is characterized by both an atypical pneumonia and efficient nosocomial transmission. This report summarizes epidemiologic features of this outbreak in Singapore, including the influence of super spreaders * and the national prevention and control straregy. +

On March 6, 2003, MOH was notified about three persons who had traveled to Hong Kong during late February and were admitted during a 5-day period to local hospitals for pneumonia (1). These patients included Case I and a traveling companion. They had been guests at Hotel M (Kowloon, Hong Kong) on February 20 and 21, coinciding with the stay of a person with SARS who transmitted disease to at least 13 guests (2). On March 14, MOH was notified about six persons, including two health-care workers (HCWs), who were admitted to Tan Tock Seng Hospital (TTSH) for atypical pneumonia; all had close contact with Case 1. Since late March, the outbreak has been characterized by nosocomial transmission caused by persons who were not immediately recognized as having SARS. The first major extension of this illness outside the health-care setting was from a recent probable SARS patient to two taxi drivers and the patient's coworkers in a wholesale market.

In Singapore, suspect and probable cases are identified and reported by using a modification of the WHO case definition that expands contact to include any health-care setting. Surveillance for suspect cases includes any fever and/or respiratory symptoms among HCWs, clusters of cases of community-acquired pneumonia, unexplained respiratory deaths, and individual cases with no contact but that are clinically suspicious for SARS. An independent, hospital-based surveillance system is being established for communityacquired pneumonia, but the threshold to monitor and investigate any unexplained febrile illness or pneumonia with laboratory testing for SARS-CoV and other etiologies is increasingly low. Source and contact tracing is conducted for probable cases and highly suspicious suspect cases on the basis of epidemiologic and clinical criteria. All persons who were household, social, hospital, and occupational contacts during the 10 days before symptom onset are traced to identify the source of infection; such c ontacts identified during the period from symptom onset to hospital isolation are traced to identify exposed persons for home quarantine.

Summary Statistics

As of April 30, a total of 201 probable cases of SARS and 722 suspect cases have been reported (Figure 1). The date of isolation of the last aurochrhonous probable case was April 28. Of the 201 persons with probable SARS, the median age was 36 years (range: 4-90 years; one was aged <12 years); 132 (66%) were female. The probable patients are Singaporean (81%), Filipino (8%), Chinese (5%), Indonesian (3%), Malaysian (2%), and Indian (1%). All persons with probable cases were hospitalized; 143 (7 1%) have been discharged after a median hospital stay of 11 days (range: 3-46 days). Twenty-two (11%) patients have required mechanical ventilation, and 25 have died (case-fatality proportion: 12.5%). Of the decedents, the median age was 53 years (range: 24-90 years), 14 (56%) were male, and 96% were Singaporean. A total of 26 persons with probable SARS have had virus, viral nucleic acid, or antibody to SARS-CoV detected in body fluids. Of the 84 (42%) HCWs with probable SARS, 49 were nurses; 13, physicians; and 22, pe rsons with other occupations (attendants, radiographers, housekeepers, a porter, and a cleaning supervisor); no SARS cases have been reported among laboratory workers or pathologists. Of the eight travelers who acquired probable SARS outside Singapore, one (Case 1) has resulted in additional cases.

For 21 probable cases with well-defined point exposures, the mean incubation period was 5.2 days ([+ or -]2.5 days) (median: 5 days; range: 1-10 days; 95th percentile = 9 days). For 94 probable cases, the mean incubation period was 5 days (+/-2.5 days) (median: 4.3 days; 95th percentile = 9.7 days) using the midpoint for well-defined exposures. A total of 172 probable cases are linked through chains of transmission to Case 1 (Figure 2). Two (1%) cases do not have an associated link with another probable case. Twelve (6%) patients have a link to a health-care facility; of these, eight are HCWs, and four are visitors. Of these 12 cases, six might be associated with two inpatients who have yet to be included in the case count; preliminary retrospective investigation suggests that these inpatients have probable cases of SARS. One probable case and seven other probable cases associated with it are linked to the wholesale market cluster. On the basis of surveillance reports, 153 (76%) infections were acquired in a health-care facility; the remainder either have household, multiple, or unknown exposures. Overall, 162 (8 1%) probable SARS cases had no evidence of transmission to other persons with clinically identifiable illness (Figure 3).

Super Spreaders of SARS

Five persons with probable SARS cases have been categorized as super spreaders of SARS. These patients appear to have infected [greater than or equal to]10 HCWs, family and social contacts, or visitors to the health-care facilities where the patients were hospitalized.

Case 1. A person aged 22 years visited Hong Kong for a shopping trip and resided at Hotel M during February 20-25. On February 25, the visitor developed a fever with a dry cough and, on March 1, was hospitalized on Ward 5A of TTSH with a patchy infiltrate on chest radiograph. The patient's platelet count was 105,000/[mm.sup.3] (normal: 130,000-150,000/[mm.sup.3]); a white blood count was 3,800/[mm.sup.3] (normal: 4,000/[mm.sup.3]). On March 4, the patient was transferred to the intensive-care unit (ICU) because of decreased blood oxygen saturation. During March 6-11, the patient was admitted to and remained in isolation on Ward 5A. On March 11, the patient was transferred to Ward 8A. This patient was directly linked to probable SARS infection in 21 persons (nine HCWs and 12 family members and visitors) and to suspected SARS in three persons. Of the ill family members and visitots, the patient's mother, father, and a visitor died. SARS-CoV infection in this patient was confirmed by virus isolation, nucleic aci d testing, and serology.