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Update: Outbreak of Severe Acute Respiratory Syndrome --- Worldwide, 2003

CDC and the World Health Organization (WHO) are continuing to investigate the multicountry outbreak of unexplained atypical pneumonia referred to as severe acute respiratory syndrome (SARS) (1). Pending development of confirmatory laboratory testing capacity, CDC's interim suspected SARS case definition (2) is based on clinical criteria and epidemiologic linkage to other SARS cases or areas with community transmission of SARS. This case definition will be updated periodically as new information becomes available. Epidemiologic and laboratory investigations of SARS are ongoing. As of April 2, 2003, a total of 2,223 suspected and/or probable SARS cases have been reported to WHO from 16 countries, including the United States (3,4). The reported SARS cases include 78 deaths (case-fatality proportion: 3.5%). This report summarizes SARS cases among U.S. residents and surveillance and prevention activities in the United States.

Descriptive Epidemiology

As of April 2, CDC had received 100 reports of suspected SARS cases (Figure) from 28 states; 81 (81%) cases occurred among adults (Table). Of these 100 suspected cases, 94 (94%) persons had traveled within the 10 days before illness onset to the areas listed in the case definition (revised on March 29 to include all of mainland China as an area with documented or suspected community transmission), four had household contact with a person with suspected SARS, and two were health-care workers (HCWs) who provided medical care to a patient with suspected SARS. Manifestations of SARS have been relatively less severe among patients in the United States than among those reported elsewhere. A majority of U.S. patients had normal chest radiographs, and 23 (23%) were reported to have pneumonia or respiratory distress syndrome on chest radiograph, thereby meeting the WHO case definition of a probable case (4). As of April 2, of the 40 (40%) patients who were hospitalized for >24 hours, 13 (33%) remained hospitalized; one patient had required mechanical ventilatory support, and no deaths have been reported.

Reports on the clinical status of suspected SARS cases are being received by state health departments and CDC, and household and HCW contacts are being monitored for the possibility of secondary transmission. Since SARS investigations in the United States began, some persons believed initially to have suspected SARS have been excluded on the basis of more complete clinical histories (e.g., no documented fever or respiratory symptoms) or because of testing results that indicated other etiologies. Alternative diagnoses have included infection with influenza virus, respiratory syncytial virus, Haemophilus influenzae, Streptococcus pneumoniae, and Staphylococcus aureus. Community transmission of SARS has not been identified in the United States; transmission to HCWs has been observed in one cluster involving two HCWs, compared with numerous reports of possible transmission to HCWs in other countries (5--7).

Enhanced Surveillance for SARS Related to Travel

As a precautionary measure, WHO has recommended that persons traveling to Hong Kong and Guangdong Province of China consider postponing all but essential travel. CDC has issued a travel advisory recommending that persons planning nonessential or elective travel to mainland China, Hong Kong, Hanoi, or Singapore consider postponing such travel until further notice. To detect possible SARS cases among travelers returning to the United States from these areas, CDC and state and local health authorities have implemented enhanced surveillance. Since March 16, notices (available in English, Chinese, Japanese, Korean, and Vietnamese) have been provided to approximately 220,000 passengers arriving in the United States on airline flights originating from China, Vietnam, and Singapore to inform disembarking passengers and crew about SARS. Persons disembarking from these countries are urged to monitor their health for 10 days after return, to seek medical care if they develop fever of >100.5º F (39.0º C) and cough or difficulty breathing within 10 days of travel, and to inform their health-care providers about recent travel to regions where SARS cases have been reported.

Laboratory Investigations

Efforts are ongoing to characterize further the role of a previously unrecognized coronavirus in SARS. Polymerase chain reaction--based assays, isolation studies, electron microscopic studies, and histologic studies are being developed to detect virus in specimens from patients with suspected SARS. Indirect immunoflorescence antibody assays and enzyme immunoassays to anti-coronavirus antibody as an indicator of infection have been developed and are being applied to specimens from suspected SARS patients. Laboratory studies at CDC and other laboratories in the WHO-organized SARS Laboratory Network have detected this new coronavirus in SARS patients, which is consistent with an etiologic role in this disease. CDC has detected human metapneumovirus from one SARS patient, and other laboratories also have detected metapneumovirus from SARS patients (6). The role of these viruses in the pathogenesis of SARS is unclear.

Reported by: CDC SARS Investigative Team; LM Fox, MD, EIS Officer, CDC.

Editorial Note:

The number of SARS cases, and the number of countries reporting such cases, continues to increase worldwide. Transmission within hospitals and households continues in some areas, and transmission within communities (e.g., Hong Kong) continues to be reported. In the absence of a complete understanding of how SARS is transmitted, efforts to limit transmission in the United States have focused on early identification of potential cases through surveillance and implementation of infection-control measures in health-care settings and the community.

CDC has developed interim infection-control guidelines for use in U.S. health-care and household settings (8). These recommendations are based on experience in the United States to date and will be revised as more information becomes available. Infection-control practitioners and clinicians providing medical care for patients with suspected SARS should consult these guidelines frequently to keep current with recommendations.

Transmission in health-care settings has been documented in several countries. Transmission to HCWs appears to have occurred primarily after close contact with symptomatic persons before recommended infection-control precautions for SARS were implemented. Because a primary strategy to reduce transmission in health-care settings is early recognition and isolation of patients who might have SARS, triage practices in hospitals and ambulatory-care settings might require reevaluation. CDC guidelines for triage of potential SARS cases are available at

In the United States, decisions to admit persons with suspected SARS to health-care facilities should be based on clinical criteria. Patients with suspected SARS who are discharged should limit interactions outside the home and not go to work, school, out-of-home child care, or other public areas until 10 days after resolution of fever and respiratory symptoms. Additional guidance for these patients is available at

The majority of U.S. residents with SARS have recovered or stabilized clinically without specific antiviral therapy. The U.S. case-fatality proportion is lower than that reported in some other countries (3). Possible explanations for this include differing case definitions among countries or differences in the sensitivity of surveillance, leading to identification in the United States of patients with less severe or early manifestations of infection or of a larger proportion of patients with other respiratory illnesses. Until confirmatory laboratory testing is available, the case definition will include clinical criteria more likely to identify potentially infectious persons. Various therapies, including antiviral agents (e.g., oseltamivir or ribavirin) and corticosteroids, have been administered to SARS patients, but the efficacy of these therapies has not been determined.

Health-care providers of patients whose illness is consistent with the case definition for SARS should continue diagnostic evaluation for other causes of respiratory illness and, when appropriate, empiric therapy that includes activity against organisms associated with community-acquired pneumonia of uncertain etiology, including both typical and atypical respiratory pathogens (9). Health-care providers who report suspected SARS cases should notify their state health departments if these patients receive confirmatory testing that indicates a diagnosis other than SARS. Information on suggested diagnostic testing and evaluation for persons with possible SARS is available at

The potential for transmission of SARS during airline travel is unknown. Transmission of other infectious agents (e.g., Mycobacterium tuberculosis) during air travel has been demonstrated (10). When an airline flight crew reports a passenger with respiratory illness, quarantine officials might board the aircraft on arrival in the United States to assess whether the passenger's symptoms match the case definition of SARS and give the passenger information about following up. If a passenger with suspected SARS is identified after passengers have disembarked, public health authorities will work with the airline to contact passengers and crew for information about the development of an illness suggestive of SARS. Although ill travelers have spread SARS rapidly across international borders, the proportion, if any, of persons who acquired SARS during international travel as a result of in-flight transmission is unknown.

Despite vigorous efforts to identify and isolate suspected cases, reducing transmission of the etiologic agents of SARS might be difficult. Understanding the epidemiology of respiratory pathogens such as those that cause SARS is challenging; approximately 40%--60% of persons with pneumonia do not have a defined etiology, even when extensive testing for known respiratory pathogens is attempted (9). Minimizing transmission will require sustained attention to infection-control interventions within health-care settings and the community. The development of laboratory testing techniques to identify infected persons rapidly will be an important step toward understanding and reducing transmission of SARS.


  1. CDC. Outbreak of severe acute respiratory syndrome---worldwide, 2003. MMWR 2003;52:226--8.
  2. CDC. Severe acute respiratory syndrome (SARS) updated interim case definition. Available at
  3. World Health Organization. Cumulative number of reported cases of severe acute respiratory syndrome (SARS). Available at
  4. World Health Organization. Case definitions for surveillance of severe acute respiratory syndrome (SARS). Available at
  5. Tsang KW, Ho PL, Ooi GC, et al. A cluster of cases of severe acute respiratory syndrome in Hong Kong. N Eng J Med. Available at
  6. Poutanen SM, Low DE, Henry B, et al. Identification of severe acute respiratory syndrome in Canada. N Eng J Med. Available at
  7. World Health Organization. Severe acute respiratory syndrome (SARS). Available at
  8. CDC. Interim guidance on infection control precautions for patients with suspected severe acute respiratory syndrome (SARS) and close contacts in households. Available at
  9. Bartlett JG, Dowell SF, Mandell LA, File TM Jr, Musher DM, Fine MJ. Practice guidelines for the management of community-acquired pneumonia in adults. Clin Infect Dis 2000;31:347--82.
  10. Kenyon TA, Valway SE, Ihle WW, Onorato IM, Castro KG. Transmission of multi-drug resistant tuberculosis during a long airplane flight. N Engl J Med 1996;334:933--8.


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Figure 1
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