There is a risk that vaccines could worsen SARS infection Credit: © Corbis

US clinical trials of experimental SARS vaccines could begin within a year. But researchers are warning against rushing into human tests.

Teams around the world are racing to develop a vaccine to fight severe acute respiratory syndrome, in case the disease resurfaces. Many groups are already assessing their experimental vaccines in animals.

Results of one of the first such trials is published today. A team led by Andrea Gambotto of the University of Pittsburgh School of Medicine, Pennsylvania, has shown that six rhesus macaque monkeys injected with a SARS vaccine pumped out antibodies and immune cells that fight the coronavirus responsible for the disease.1

At least three other groups are thought to have already taken the next key step: showing that their vaccines actually protect animals such as monkeys or ferrets against SARS infection.

And Chinese government officials announced in late November that they were ready to begin human trials of a SARS vaccine made by Beijing-based company Sinovac Biotech.

But SARS vaccines, like those against some related animal coronaviruses, could actually worsen the infection. "It's important that we don't rush in," says Gary Nabel, who heads the vaccine-research centre at the National Institutes of Health in Bethesda, Maryland.

Fuel or fight

Vaccines trigger the production of antibodies that latch onto viruses. Vaccines also prompt white blood cells called macrophages to eat and destroy the antibody-tagged viruses. But in some cases, coronaviruses replicate inside the macrophages themselves ? fuelling, rather than fighting, the infection. This phenomenon is called antibody-mediated enhancement.

So far, there is no sign that the SARS coronavirus reproduces in macrophages, says Chris Olsen, who studies animal vaccines at the University of Wisconsin-Madison. "But it's important to document," he says.

It's happening at an unprecedented rate Gary Nabel , NIH

To check whether antibody-mediated enhancement will be a problem for SARS jabs, Nabel advises researchers to stick to small clinical trials at first. "None of us wants to see a vaccine go forward that has any hint of doing harm," he says.

Alternative therapy

There are many different routes to a SARS vaccine. Gambotto's team packaged three SARS genes, including one that helps the virus breach human cells, into a harmless virus. Once in the body, the virus makes proteins that trigger an immune response.

The Sinovac Biotech vaccine takes a different approach: using a whole, dead strain of the SARS coronavirus. Other groups are using naked SARS DNA.

For now, it is unclear whether one or a combination of these tactics will ultimately prove most effective. Nabel predicts that as many as five candidate vaccines could be in small-scale clinical trials within a year. "It's happening at an unprecedented rate," he says.