At least 10 candidate vaccines are under development in Asia, Europe and the United States, according to Marie-Paule Kieny, director of the Initiative for Vaccine Research at the World Health Organisation (WHO).

Even so, "it will take a few years" before any of these are proven safe and effective for the public, she told AFP.

Heading the pack is a fast-tracked prototype from China that uses a killed virus to provoke an immune response.

The trial vaccine has been successfully tested on monkeys and last month, the Chinese state media announced it would be assessed for safety on 30 human volunteers, a process that would span three months.

This is Phase I of the human testing procedure. Phases II and III involve much larger groups of recruits to test again for safety but also for efficacy, before the results can be reviewed and the vaccine licensed.

"Making a vaccine that's safe and can be put into people is usually something that takes a minimum of five years," said Maria Zambon, head of the flu laboratory at Britain's Health Protection Agency.

The time can be shortened if there is a worldwide peril.

But express-testing new drugs can also come at the cost of safety, as experiences showed with the very first anti-HIV virals, which for many patients had grim side effects.

The bad news for SARS: The scare factor that drives vaccine funding and public-health policy has receded.

The big outbreak was halted in the middle of 2003 by the simple, time-honoured approach of isolating patients and quarantining individuals who had been in contact with them.

By the end of the epidemic, cases had been reported in 32 countries, infecting some 8,000 people and killing close to 800, nearly half of them in China.

The crisis also cost billions of dollars in lost industrial production, ravaged tourism and decimated airline schedules, thus highlighting the clear need for a vaccine.

But, Zambon said, containment of the outbreak meant corporate interest in a SARS vaccine -- a long and costly quest with an uncertain reward -- has "definitely, definitely fallen back."

Despite this, funding lines have been opened up by the US National Institutes of Health (NIH) and the European Union's executive Commission, dangling money for joint public-private sector initiatives.

"I would expect to see in the next year or two a number of different academic sectors/companies reporting their progress, be that in animal vaccine development or vaccine for SARS in humans," Zambon said.

The "killed virus" approach adopted by China is a classic, swift design which hopes to induce a broad spectrum of antibodies and be long-lasting, said Sylvie van der Werf, a professor at the Pasteur Institute in Paris.

The downside, though, is that sometimes a killed virus vaccine can act on the immune system in unexpected ways.

"This is what we saw in a vaccine for feline infectious peritonitis. It encouraged infection rather than protection," she said.

There remain many unknowns about the coronavirus that cause the disease.

Among them are the animal "reservoir" where the virus naturally holes up; how the SARS virus works once it enters the body; if, like the flu virus, the SARS agent mutates steadily or suddenly; and why some individuals are mysteriously at greater risk to it than others.

Understanding that will help the lab engineers who are seeking a formula that will prime antibodies and killer cells against the virus.

It will also help decision-makers who must decide whether vaccination efforts would be better directed at animals rather than humans, or at individuals who are particularly vulnerable to the virus or most at risk of transmitting it, Zambon said.

TERRA.WIRE