Health Officials Struggle to Understand SARS

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Until now, though, coronaviruses that infect humans have not been lethal. They inflict the minor misery of the common cold. But will SARS turn the coronas into mass killers?

If the virus exists in a wild widespread host—like the West Nile virus carried by mosquitoes—then SARS could be a perennially recurring problem, according to Lai.

If the host is a domestic animal—like the chicken responsible for the Avian Flu virus of 1997—then the source of the virus could be eliminated. The Avian Flu effectively stopped after the Hong Kong government ordered the slaughter of chickens in the area.

Lai suspects that the SARS virus lurks in a wild animal. "It probably doesn't bother domestic animals or we would have come across it before," he said.

If authorities cannot stem the spread of the disease in people, SARS could become endemic in humans—like the flu, said Alison Galvani, a research fellow at University of California, Berkeley, who models the evolution and epidemiology of infectious agents.

Conversely, if people are the only carriers of SARS, "honest reporting of SARS cases and stringent quarantines could [potentially] stop the virus in its tracks," Lai said.

Knowing a Virus, Predicting an Epidemic

"Making a vaccine could be quite straightforward, if it is needed," Lai said. Vaccines already exist for swine and chicken coronaviruses.

But it isn't easy to gauge whether SARS, or any virus, can trigger an epidemic.

"Generally, we can't predict virulence or even whether a virus can infect humans," said Stephen Morse, director of the Center for Public Health Preparedness at Columbia University in New York City and editor of the book Emerging Viruses.

"We've been working with influenza for over 50 years and it can still evade our best attempts."

"In many cases of the influenza virus we don't understand the molecular properties that cause high mortality," said Nancy Cox, chief of the influenza branch at the CDC.

Producing a new influenza vaccine every year requires year-round surveillance. Flu infects between 600 million and 1 billion people around the world every year. Disease centers from 83 countries collect about 175,000 virus samples.

From those the researchers pick the three worst threats as the basis for next year's vaccine. Scientists have correctly predicted the three predominant strains of influenza virus for eight of the past ten years.

"Outbreaks are like wildfires," Morse said. "Some fires smolder and are easily extinguished. Others rage out of control, destroying everything in their path. The WHO and the CDC are like the fire department and they need to investigate all outbreaks because you never know which way they could go."

Morse points out our long history with influenza. SARS is only a few weeks old. Tracking and containing viruses, and predicting what they will do, Morse said, "is as much an evolving science as an evolving art."



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