The global response to SARS.

The global approach to the Severe Acute Respiratory Syndrome (SARS), coordinated by the World Health Organization (WHO), has been aimed at sealing off opportunities for further spread, both within countries reporting cases and internationally. Experiences in a growing number of countries indicate that the disease can be contained, thus supporting the WHO overall objective: to prevent SARS from becoming widely established as another new disease in humans.

In the absence of a vaccine, the most effective way to control a new disease such as SARS is to break the chain of transmission from infected to healthy persons. In almost all documented cases, SARS is spread through close face-to-face contact with infected droplets when a patient sneezes or coughs. Three activities--case detection, patient isolation, and contact tracing--can reduce the number of people exposed to each infectious case of SARS and eventually break the chain of transmission, Detection aims to identify SARS cases as soon after onset of illness as possible and once identified, the next step is to ensure their prompt isolation in a properly equipped facility and management according to strict procedures of infection control. Tracing--the detective work--involves the identification of all close contacts of each case and assurance of their careful follow-up, including daily health checks and possible voluntary home isolation.

Together, these activities limit the daily number of contacts possible for each potentially infectious case. They also work to shorten the amount of time that lapses between onset of the illness and isolation of the patient, thus reducing opportunities for the virus to spread to others. The effectiveness of these measures is reflected in an important indicator of disease transmission--the so-called "effective reproduction number".

This is the average number of new cases generated by each existing case. If each new SARS patient infects more than one person, then the number of new cases will increase with time. If each new SARS patient infects only one further person, then the number of new cases will remain static. However, if each new SARS patient infects on average less than one further person, then the number of new cases will fall and the outbreak will eventually die out.

Containing the epidemic remains a WHO priority, both in its support to individual countries and globally. Singapore has instituted comprehensive procedures for contact tracing and isolation, and these measures appear to be working. The so-called "time to isolation" had been reduced from a high of three days in the early phase of the outbreak to 1.4 days, thus reducing by half the amount of time infected persons can expose others to the SARS virus. These measures are being applied elsewhere, with outbreaks coming under control in several areas.

WHO is now collecting the data needed to systematically assess the effectiveness of these and other control measures being implemented worldwide and will strengthen its recommendations accordingly. Nonetheless, as SARS has clearly demonstrated, a single case admitted to an unprepared hospital can ignite a new outbreak. While trends are moving in an encouraging direction in many areas, even greater focus on control measures is needed if SARS is to be contained globally. WHO experiences with outbreaks of Ebola haemorrhagic fever have repeatedly shown that the initial stages of containment are the most dangerous time to start lowering the level of control. "We cannot become complacent", says Michael J. Ryan, Coordinator of the WHO Global Alert and Response Programme. "In fact, this is the time to intensify our efforts."