about Epidemiology & the department

Epidemiology academic information

Epidemiology faculty

Epidemilogy resources

sites of interest to Epidemiology professionals

Last Updated

29 Jul 2003

Source: Washington Post, July 20, 2003

The SARS Triumph, And What It Promises

By David Brown

Las Vegas bookmakers haven't yet started taking bets on whether severe acute respiratory syndrome (SARS) will reappear in the human population. But it's probably only a matter of time. The question is one of the more interesting and important ones hanging over the planet this year, and the answer will probably be known by Christmas.

It's remarkable that such a virulent and feared disease might be gone for good, less than six months after it emerged and just three months after it brought the world's fastest-growing economy to a near halt and turned the cotton surgical mask into China's national symbol. Yet, earlier this month, the World Health Organization (WHO) announced that the SARS virus's chain of transmission had been broken. No one had become ill for two consecutive 10-day incubation periods following the most recent case, which was identified in Taiwan on June 15. The disease, in short, had ceased reproducing itself.

It is possible we have just witnessed the eradication of a disease for only the second time in human history. Eradicating the first -- smallpox -- took 10 years, 1967 to 1977. It's one of the great achievements of human cooperation. The next target -- polio -- has proved a more elusive quarry. A campaign to rid the world of it began in 1988 and those in charge desperately hope to finish by the end of 2005, five years past its original deadline.

The fact that between those epochal efforts a new disease emerged, announced itself by killing 15 percent of its victims, took hold most strongly in the world's most populous nation, had at least one victim on six continents, and was stopped in its tracks so quickly -- well, it's an improbable scenario, to say the least.

The SARS success is a product of luck, skill and historical moment. The luck lay in certain biological traits of the SARS virus, and possibly in the Asian origin of the outbreak, both of which favored its control. The skill was evident in the wielding of some of public health's oldest tools -- case-finding, quarantine and isolation -- on a massive scale. The historical moment featured new methods of instantaneous exchange of information and a spirit of globalization that, with the assistance of fear and trembling, promoted cooperation to an extreme degree.

Of course, it may be that SARS will come back in the cooler, drier months of winter, as most respiratory viruses do. This requires that it still be out there someplace, either in an animal reservoir or in human beings carrying it or transmitting it in undetectable form. (Most experts think one of those possibilities is likely to be true.) But even if it does return, the experience of the last few months has been epochal in its own right.

The outcome might have been different if the SARS coronavirus had been different. When rumors emerged from southern China in January, little was known other than that people were dying of a new, slowly progressive pneumonia. The cause and contagiousness of the disease, who was at risk and why -- none of this was clear. Infectious disease experts now realize that while SARS is deadly, it isn't terribly contagious. Transmission generally requires close, face-to-face contact with the ill, which is why hospital workers are so vulnerable. The average number of people each SARS patient infected in the early, uncontrolled stage of the epidemic was three. (The estimate comes from the experience in Hong Kong and Singapore.) This key number -- called the "basic reproductive number" by epidemiologists -- is a product of a pathogen's biological nature and, to a lesser extent, the social setting in which it's transmitted. For measles, the basic reproductive number is 11 to 14; for chicken pox, 10 to 12; for polio, 5 to 7.

SARS had an opportunity to flash into a population-wide epidemic in southern China between its appearance late last year and the initiation of serious public health interventions in April. That this didn't happen suggests the virus may not have the pandemic potential of, say, a wholly new strain of influenza, which could spread around the world in months, if not weeks.

One characteristic of SARS that helps explain why it failed to run out of control is that a person is most likely to transmit the virus a week or more into the illness. In most respiratory infections, the amount of virus in the nose and throat is highest when symptoms begin. In SARS, it's the opposite: Virus levels don't peak until about 10 days after people start feeling sick. This trait proved to be as lucky as it was unusual. It magnified the benefit of identifying SARS patients early -- and the incentive for undertaking the search.

Public health authorities in China, Hong Kong and other Asian sites of the outbreak expended great effort to shorten the time that SARS patients were in circulation . They did this by exhorting people to seek medical attention at the first sign of illness, and by screening millions of people for fever, which is present in 95 percent of cases. Every day that was shaved off the interval between symptoms and diagnosis was one day less that a SARS patient could infect someone else.

In Hong Kong, that interval fell from about five days to two days over the course of the epidemic. In Beijing, it averaged six days before April 21, and was probably even longer in southern China in the months immediately following the first cases last November. That third week in April marked the turning point in the government's response. Between then and the last case in June, the average time from first symptom to diagnosis and isolation was two days. In the final few weeks, it was one day, reports Anne Schuchat, a physician and epidemiologist for the Centers for Disease Control and Prevention (CDC) who spent the late spring in WHO's Beijing office.

"They had just massive community mobilization and awareness," she said in an interview.

There is no precedent in public health for a reduction of this magnitude in the average interval of infectiousness, says Roy M. Anderson, an epidemiologist at Imperial College in London. It literally took the SARS coronavirus out of circulation. At the same time, hundreds of thousands of people who'd had contact with possible SARS patients were quarantined, taking them out of circulation as well.

Beijing, which has the best-tabulated statistics at the moment, reports that as of mid-June, 30,172 people had been quarantined sometime during the epidemic, either at home or at work, for two weeks. The number of people with symptoms who were hospitalized isn't known at the moment, but it's certainly more than double the final tally of 2,521 "probable" cases in the city.

The Chinese SARS experience is likely to go down in history as the gold standard of what can be done in an epidemic if there's enough motivation and no one objects to overkill. Once the nation's leaders got past their initial denial of the problem, China's authoritarian political structure, its culture that emphasizes group values and its enormous work force greatly aided control of the disease. Whether other nations would go to the same ends is an interesting question, and one that only a similarly frightening epidemic inside their borders could answer.

In Beijing, about 60 fever clinics were established nearly overnight. The Chinese military assembled a 1,000-bed modular isolation center for SARS cases in the Beijing suburb of Xiaotangshan in eight days and nights. As many as 7,000 workers were on the site some days, according to press reports. Jiangsu Province, north of Shanghai, had only seven probable cases, but at one point had more than 10,000 people under quarantine. Many distant villages barred entry to people who'd traveled through Beijing, or even near it.

In the capital city, residents could barely escape having their temperature checked at least three or four times a day. Anderson, visiting hospitals and public places late in the epidemic, one day had his checked 16 times. "Once the government got its mind around the problem, it applied it with great rigor," he said, referring to China's strategy comprised of quarantine, contact tracing and isolation of cases. "It would have been very difficult to do the same in North America or Europe. You have to convince people that death is right around the corner before you limit their civil liberties. Otherwise, you end up with court cases."

Anderson helped advise the British government during the 2001 foot-and-mouth disease epidemic in the United Kingdom. Control strategies primarily affected animals, not people. Nevertheless, lawsuits arising from that outbreak are still being adjudicated, he said.

Although there wasn't (and still isn't) any rapid method of diagnosing SARS, and the symptoms were indistinguishable from those of a half-dozen other illnesses, the eradicators had something else going for them. SARS emerged into a world endowed with the Internet and cellular telephones. This allowed nearly instantaneous communication about spread of the disease, prevention and treatment strategies, and research findings. The smallpox eradication campaign -- and the polio one, for most of its duration -- took place in a different world.

J. Michael Lane worked in CDC's smallpox eradication office from 1964 to 1980 and headed it for 10 years. For a time he labored in India and Bangladesh, investigating outbreaks, isolating new cases and vaccinating contacts. "You could have a team doing the same thing 10 miles away from you. You might not hear about them for three or four days, until someone on a bicycle rode over and told you the story," he recalled recently. "WHO was very happy if they were getting weekly telegraphs from countries, and the information they got was the previous week's. So case counts were always two weeks out of date. Now, if there's a big outbreak of something in Toronto, everyone knows in 24 hours."

Instant communication has magnified the authority of WHO. That Geneva-based entity has dozens of functions, and very different roles in different countries, depending on their degree of economic development. But until the annual meeting in May of its member countries, WHO had no authority to forcibly investigate disease outbreaks. Even now its alerts and travel advisories are rhetorical and unenforceable.

In the SARS outbreak, however, WHO was the natural hub to which information flowed. People turned to it each morning for updates. It helped organize a network of laboratories that produced a sizeable body of scientific knowledge about the disease within two months. (Dissemination of that knowledge, in turn, was speeded by the willingness of medical journals to publish papers online, long before they'd appear in print.) Like the natural if unelected leader of a crowded lifeboat, WHO gave orders that no one wanted to be the first to disobey.

The SARS outbreak will be teaching the world lessons for a long time -- not only about virology, epidemiology and medicine, but about institutional behavior, economics and politics. It may also turn out to be an unplanned dress rehearsal for an infectious cataclysm that experts have actually been predicting -- the emergence of a wholly new strain of influenza virus.

This will arise from a big "shift" and not just a small "drift" in the genetic identity of a circulating flu virus. Such shifts occurred in 1918, 1957 and 1968. The 1918 pandemic killed about 50 million, according to recent recalculations of its toll. Two of those three new strains first appeared in China. We're due, experts say.

Few things are as contagious, or move as fast, as a strain of flu that nobody in a population has any immunity to. Bloodstream antibody studies of Americans who lived through the 1918 pandemic found that more than 95 percent had been infected by the virus. Stopping -- or even slowing -- such a force of nature would be much, much harder than stopping SARS. But the experience of the past five months suggests the world is prepared to fight back in a way it never has before.

David Brown writes about science and medicine for The Washington Post.