SARS.

Background

Severe acute respiratory syndrome (SARS) has been aetiologically linked to a new coronavirus called the SARS-associated coronavirus (SARS-CoV)[1]. Genome-sequence analyses are consistent with the hypothesis that the SARS-CoV has an animal origin, from an as-yet unknown reservoir[2]. SARS is most commonly spread by close person-to-person contact, which involves exposure to infectious respiratory droplets. Estimates of the incubation period have converged at 2–10 days[3], and modelling data from Hong Kong indicate that 95% of cases have incubation periods of up to 14 days. An incubation period of 1 day has been reported in a small number of cases[4]. Transmission is amplified in healthcare settings, although community transmission also occurs[2]. The sentinel event for the international spread of SARS has been attributed to environmental contamination in a hotel[5], and point-source outbreaks have occurred through faecally contaminated aerosols in housing estates in Hong Kong[6].

Global distribution of probable SARS cases by country

The first known case of SARS was identified in Guangdong Province, China, in November 2002 (Ref. 7), and spread of the disease to other parts of the world was associated with international air travel. In March 2003, the World Health Organization (WHO) issued a global alert and a travel advisory to contain the disease[8,9]. As of 26 September 2003, the WHO had received reports of more than 8,000 probable cases of SARS and 774 deaths due to SARS, giving a global fatality rate of 9.6% (Ref. 10). Areas with the highest burden of disease were identified as China, the Hong Kong Special Administrative Region of China, Taiwan, Singapore, Canada and Vietnam. These countries and provinces experienced sustained local transmission between healthcare workers — who formed a large proportion of the early cases.

The WHO, together with its Global Outbreak Alert and Response Network (GOARN) partners, brought together an international team of health professionals — including clinicians, virologists, epidemiologists, public health professionals and environmental engineers — to study and contain the disease. This international response enabled the rapid identification of the infectious agent, its mode of transmission and the development of diagnostic tests. Traditional public health measures (for example, case detection and isolation, stringent infection control, contact tracing and surveillance) were used globally to successfully break the chain of transmission[11]. SARS remains a public health challenge for several reasons, including nonspecific symptoms and signs, a lack of diagnostic tests that can be used to reliably detect SARS-CoV in the first few days of illness, the risk of superspreading events (which fuelled large disease clusters in the 2003 outbreak), the lack of a vaccine or agreed treatment modalities and limited data on the ongoing risk of interspecies transmission. Re-emergence of SARS on a large-scale is possible and preparedness of public health and healthcare systems is important to prevent and control the disease.

Recent developments

Despite the rapid progress that has been made in the global understanding of SARS, many questions remain, including the risk of a re-emergence of SARS. The WHO has established a SARS Research Advisory Committee to review the evidence, determine gaps in knowledge and make recommendations on the public health research priorities[12]. The recommendations made include further elucidation of the transmission dynamics, such as interspecies transmission; a risk-benefit analysis of therapeutic modalities used during the outbreak; the development of protocols for future multicentre clinical trials, diagnostic quality assurance and laboratory biosafety; and analysis of the social and economic impact of SARS and the public health measures used to control it. The minimum level of global preparedness that is required to reduce the risk of emerging infectious diseases and mitigate their impact was also discussed.

The WHO SARS aetiology network sequenced the SARS-CoV genome and developed several diagnostic tests with unprecedented speed. However, available tests still lack sufficient sensitivity early in the disease, and further evaluation of their performance in different epidemiological situations is required. Research is also progressing on SARS vaccines. Inactivated, live, attenuated and subunit vaccines are among those under investigation, and the first clinical trial of an inactivated SARS vaccine is expected to begin early in 2004 (Ref. 13).

The WHO established global surveillance for 'suspect' and 'probable' SARS cases on the basis of clinical and epidemiological criteria. These have been modified in the post-outbreak period to reflect the need for ongoing vigilance with a low, but non-zero, risk of SARS in most countries, and to incorporate laboratory case definitions. Revisions are also being made to the International Health Regulations, which provide the legal framework for the global surveillance and reporting of infectious diseases and a mechanism by which measures to prevent international spread can be enforced[14].

Conclusions and future outlook

The SARS epidemic severely stretched the national health systems of affected countries and tested the capacity of the WHO and its GOARN partners to respond quickly and effectively to a global health emergency. SARS has shown the importance of international collaboration to deal with an emerging infectious disease threat and the invaluable position that the WHO occupies in global health security. With its privileged access to the member states, the WHO was able to undertake global coordination of the SARS response by facilitating the sharing of information between health professionals and global surveillance and laboratory research teams, and the mobilization of technical co-operation. The global experience gained during the outbreak has increased our readiness to implement prevention, containment and control measures quickly and effectively should SARS return.