travel. WHO regards every country with an international airport, or bordering an affected area, as at potential risk of an outbreak.

On 21 February, SARS was carried out of Guangdong Province, China by an infected medical doctor who had treated patients in his home town. He brought the virus to the ninth floor of a four-star hotel in Hong Kong. Days later, guests and visitors to the hotel's ninth floor had seeded outbreaks of cases in the hospital systems of Hong Kong, Viet Nam, and Singapore. Simultaneously, the disease began spreading around the world along international air travel routes as visitors at the hotel travelled home to Toronto and elsewhere, and as other medical doctors who had treated the earliest cases in Viet Nam and Singapore travelled internationally for medical or other reasons.

The number of probable SARS cases passed the 6000 mark on Friday 2 May, with 27 countries reporting cases from five continents. More than 400 deaths have occurred. China is reporting a cumulative total of probable cases that is approaching 4000 as each day's nationwide reporting adds at least 100 new cases. Although outbreaks in Hong Kong, Singapore, and Toronto show signs of having peaked, new cases and deaths continue to be reported. Taiwan, with a rapidly growing number of cases and deaths, is a worrisome new development.

A particularly serious threat SARS needs to be regarded as a particularly serious threat for several reasons. The disease has no vaccine and no treatment, forcing health authorities to resort to control tools dating back to the earliest days of empirical microbiology: isolation and quarantine. The virus comes from a family notorious for its frequent mutations, raising important questions about the future evolution of outbreaks and prospects for vaccine development. Epidemiology and pathogenesis are poorly understood. All available diagnostic tests have important limitations. If tests are poorly conducted or results wrongly applied, patients excreting virus and thus capable of infecting others can slip through the safety net of isolation and infection control. The disease continues to show a disturbing concentration in hospital staff — the human resource vital to control. A significant proportion of patients require intensive care, thus adding to the considerable strain on hospital and health care systems. Evidence is mounting that certain source cases make a special contribution to rapid spread of infection. SARS has an incubation period that allows rapid spread along international airtravel routes.

With the notable exception of AIDS, most new diseases that emerged during the last two decades of the previous century or have become established in new geographical areas have features that limit their capacity to pose a major threat to international public health. Many (avian influenza, Nipah virus, Hendra virus, Hanta virus) failed to establish efficient human-to-human transmission. Others (Escherichia coli O157:H7, variant Creutzfeldt-Jakob disease) depend on food as a vehicle of transmission. Diseases such as West Nile fever and Rift Valley fever that have spread to new geographical areas require a vector as part of the transmission cycle. Still others (Neisseria meningitidis W135, and the Ebola, Marburg, and Crimean-Congo haemorrhagic fevers) have strong geographical foci. Although outbreaks of Ebola haemorrhagic fever have been associated with a case

fatality rate in the range of 53% (Uganda) to 88% (Democratic Republic of the Congo), person-to-person transmission requires close physical exposure to infected blood and other bodily fluids. Moreover, patients suffering from Ebola during the period of high infectivity are visibly very ill and too unwell to travel.

The SARS response To date, the global response, coordinated by WHO and strongly supported by CDC, has been designed to rapidly seal off opportunities for SARS to establish itself as a common disease. The initial emergency plan, mapped out in midMarch, called for an attack on the ground and in the "air". On the ground, WHO sent teams of experts and specialized protective equipment for infection control in hard-hit hospitals to countries requesting such assistance. In the "air", WHO used the model of its electronically interconnected global influenza network to quickly establish a similar "virtual" network of 11 leading laboratories, connected by a shared secure website and daily teleconferences, to work around the clock on identification of the SARS causative agent and development of a robust and reliable diagnostic test. This network, in turn, served as a model for similar electronically linked groups set up to pool clinical knowledge and compare epidemiological data. WHO also decided to issue daily updates on its website to keep the general and travelling publics informed and, to the extent possible, counter rumors with reliable information.

On 15 March, based on information from WHO country offices and GOARN partners, followed by risk analysis by the WHO headquarters operational team, WHO issued a rare emergency travel advisory designed to alert national authorities, medical personnel, and travelers to an emerging threat that was quickly taking on international dimensions. Global vigilance was immediately heightened, with the result that most countries subsequently reporting cases have managed, through prompt detection, isolation and good infection control, to prevent the scale of transmission experienced in the SARS "hot zones". On 2 April and again on 19 April, WHO issued the toughest travel advisories in its 55-year history when it recommended postponement of all but essential travel to designated high-risk areas.

WHO teams continue to provide operational support and specialized expertise in the most seriously affected areas. Requests for additional country assistance continue to be received, most notably from authorities in China. Abundant additional support is available to all through information posted at the WHO website (www.who.int/csr/sars). Guidance ranges in nature from forms for collecting and reporting data, through guidelines for clinical management and infection control in hospitals, to the materials for local production of diagnostic tests. The evolution of the outbreak is constantly and closely monitored and daily updates are posted on the website. On 17 April, exactly a month after its establishment, the laboratory network announced conclusive identification of the SARS causative agent: a new coronavirus unlike any other known human or animal virus in its family. The laboratory reagents needed to calibrate, standardize and assure the quality of laboratory tests are being made available by WHO, at no cost, to laboratories designated by ministries of health.

Lessons for the future

Just as the SARS response has been guided by lessons learned during preparedness planning for the next influenza pandemic and for a possible bioterrorist attack, both of these types of potential public health emergencies will benefit from lessons learned as the international response to SARS continues.

The response to SARS has already brought to light a number of positive lessons as well as highlighted a number of challenges for future preparedness planning. The SARS experience has shown the capacity of global alerts, widely supported by a responsible press and amplified by electronic communications, to improve global vigilance and awareness at all levels, from health professionals and national authorities, to politicians and the travelling public. The quick detection and reporting of the first cases in South Africa and India are indicative of the high level of global awareness and the vigilance of the world's health systems. The present climate of high alert also helps explain the speed with which developing countries - from Namibia to Mozambique - have readied their health services with preparedness plans and launched SARS campaigns, often with WHO support, to guard against imported cases.

The SARS experience in Viet Nam has shown that immediate political commitment at the highest level can be decisive. Viet Nam demonstrated to the world how even a very poor country, hit by an especially large and severe outbreak, can triumph over a disease when reporting is prompt and open and when WHO assistance is quickly requested and fully supported.

And finally, stimulation of very rapid, high-level research has been seen clearly to be a key component of an effective response.

The key challenges to be addressed in future planning are those of surge capacity and transparency. Inadequate surge capacity in hospitals and public health systems has clearly been a major problem, especially when health care workers have themselves been victims of the disease and are the frontline troops at risk. The shortage of expert staff to coordinate national and global responses to a rapidly evolving public health emergency is also an issue needing investment and attention.

SARS is now known to have begun in mid-November in a southern province of China. Cases during the earliest phase of the SARS outbreak there were not openly reported, thus allowing a severe disease to become silently established in ways that made further international spread almost inevitable. This is the most important lesson for all nations: in a globalized, electronically connected world, attempts to conceal cases of an infectious disease, for fear of social and economic consequences, must be recognized as a shortterm stop-gap measure that carries a very high price - loss of credibility in the eyes of the international community, escalating negative domestic economic impact, damage to the health and economies of neighboring countries, and a very real risk that outbreaks within the country's own territory can spiral out of control.

The SARS experience also has some lessons about the importance of international collaboration and strong but politically neutral global leadership. Though exceptional in terms of its impact, severity, rapid international spread, and many puzzling features, SARS is only one of around 50 internationally important outbreaks to which WHO and its partners respond in any given year. The high level of medical, scientific, political, and public attention focused on SARS is helping the world to understand the severity of the infectious disease threat and the importance of international solidarity in the face of this threat. It is also helping the world to understand the importance of global leadership and of politically neutral and privileged access to all affected countries. Finally, the response to the SARS outbreak is helping the public to understand that WHO's activities of global coordination, capacity development, communications, and mobilizing expertise enable rapid response and actually save lives. To date, in the vast majority of countries, these activities have helped health authorities confronted with imported cases prevent a SARS outbreak and thus avoid the devastating consequences seen elsewhere.

Improving infectious disease surveillance and response is indeed a matter of “national security, emerging threats and international relations" as this Subcommittee's name implies. Global public health security will continue to require effective leadership and action at a global level by WHO and its partners.