The SARS virus, which terrorized the world in 2002, could yet inflict another epidemic. Credit: © AP

Two years ago, a lethal virus was brewing in rural China, one that was subsequently dubbed SARS, for severe acute respiratory syndrome. Now a new vaccine against the disease is being tested in humans - at a time when it is at risk of drifting off the public-health radar.

SARS shocked the world when it emerged from China in 2002, causing panic as it raced around the globe and killed nearly 800 people. It was eventually brought under control in 2003 by a quarantine of exposed people.

Since then, a few isolated outbreaks have been caught and aggressively stamped out, and the disease has largely fallen out of the spotlight. But public-health experts remain concerned that a second wave of infections could erupt, either from human contact with infected animals or by the virus escaping from laboratory samples.

"Once a virus like that makes an appearance there is no assurance it will stay dormant," says Gary Nabel, who leads the team developing the jab at the US National Institute of Allergy and Infectious Diseases in Bethesda, Maryland. The researchers yesterday unveiled their plan to test the vaccine on humans.

The vaccine is a small piece of DNA from the virus, which codes for a protein in its coat. Researchers hope that after it is injected into the body, human cells will take up the DNA and begin to manufacture the protein themselves.

The immune system should learn to recognize this foreign protein and, if it then encounters the genuine SARS virus, launch an all-out attack. Nabel's team showed earlier this year that the vaccine slashes the rate at which the virus replicates in infected mice1.

Nabel's preliminary trial, involving ten people, will test whether the jab is safe and whether it fires up the immune system. If it is successful, Nabel plans to expand the trial, with the eventual aim of stockpiling the vaccine for use in the event of an outbreak.

At least one vaccine is already undergoing clinical testing in China, using a different approach in which people are injected with the entire killed virus. It remains to be seen which strategy will prove most effective. And some groups are developing infusions of antibodies against SARS which, unlike a vaccine, could be given to someone who is already infected with the virus.

Animal aversion

Heightened control and surveillance measures in China are probably helping to avert the re-emergence of SARS, say scientists who are studying the disease. Early this year, for example, Chinese authorities ordered a mass cull of civet cats, which are thought to harbour the virus and pass it onto humans in live-animal markets.

But the threat of SARS is being eclipsed by concerns about other infectious diseases, such as avian flu, says virologist Herbert Schmitz of the Bernhard Nocht Institute for Tropical Medicine in Hamburg, Germany. Bird flu rampaged through poultry stocks in Asia this year, raising fears of a human pandemic with many millions of deaths.

Indeed, says Schmitz, there are many unknown or uncharacterized viruses lurking in animals around the world that have the potential to cause disease in humans. The threat of new ones emerging is escalating as humans invade previously isolated ecosystems and come into close contact with their animals.

Both avian flu and SARS warrant extreme vigilance, Nabel argues. "It's like saying: 'Would you rather have a 35 mm or 28 mm bullet?'," he says. "Both can kill. Because we worry about one should not mean we don't worry about the other."