Tests in China Suggest Some With SARS Don't Become Ill

N.Y. Times.com - Science

May 27, 2003

tests of people who worked with exotic animals in markets in southern China show that a significant proportion apparently had been infected with the SARS virus, suggesting that some people may become infected without becoming ill, World Health Organization officials said yesterday.

The findings are from two separate studies of workers in markets in Guangdong Province in China, and they strengthen the SARS link between animals and humans, the officials said in interviews.

At the same time, scientists at Hong Kong University announced last night that they were developing what could become the first experimental vaccine for severe acute respiratory syndrome. They said they hoped to start testing the vaccine in animals in early June.

The scientists said that human trials of any SARS vaccine would not begin until after findings of animal experiments are known, and perhaps not even then. Preliminary results of the animal experiments will not be known for at least six months, the scientists said.

Last week, scientists in Hong Kong and the Center for Disease Control in Shenzen, just across the Hong Kong border in Guangdong in mainland China, reported finding the SARS virus in three species of animals  the Himalayan, or masked, palm civets; raccoon dogs; and badgers  bought in a food market in Shenzen. That discovery suggested, but did not prove, that the SARS virus infects animals in the wild, making it virtually impossible to eradicate the disease.

One of the two new studies involved workers in the same market in Shenzen.

Dr. Klaus Stvhr, the scientific director of the W.H.O.'s investigation of SARS in Geneva, said that a team of virologists headed by Dr. Malik Peiris of Hong Kong University tested blood taken from 10 workers in the market and found antibodies to the SARS virus in 5, or 50 percent. Antibodies are specific proteins that the immune system forms when it mounts an attack against a microbe. But antibody tests obtained in such screening samples cannot determine when an individual was infected in the past. The workers with positive tests were said to be healthy and did not recall having a SARS-like illness recently, Dr. Stvhr said.

Scientists at the Center for Disease Control in Guangdong performed a similar screening study among 508 people working in a larger group of wholesale markets of exotic animals, Dr. Stvhr said. Of these, 66, or 13 percent, had antibodies to the SARS virus. No information was available on the medical histories of the infected workers, he said.

The findings are not definitive, but strengthen the idea that animals play a role in transmitting SARS, Dr. Stvhr said. The findings, he said, "suggest that the spectrum of disease is wider than what we saw" when SARS was first detected as a severe form of atypical pneumonia.

There are many reasons that the percentage of infected workers varied in the two studies and that those infected apparently did not become ill. For example, the sensitivity of the tests used by the two laboratories may have varied. Also, the amount of virus to which the workers were exposed was small, thus reducing the chances of more serious illness.

"We need to investigate it further," Dr. Stvhr said.

Dr. David L. Heymann, executive director of communicable diseases for the W.H.O., said that similar studies needed to be conducted to determine the frequency of SARS infection among people who do not work in the markets but live in the same area.

Laboratories around the world have been racing to develop vaccines that might be used to prevent SARS in the event that the main control measures of isolating people with SARS and quarantining their contacts fail to stop transmission.

The need to develop an effective human SARS vaccine became more urgent with the discovery that the SARS virus exists outside humans. The discovery also left open the possibility that infected animals might be a continued source of infection in humans.

Lennon Tsang, a spokesman for Hong Kong University, said the vaccine would be derived from an "inactivated" strain of the coronavirus that the W.H.O. says is the cause of SARS. The vaccine uses a "noninfectious" form of the virus, he said, declining to discuss whether the vaccine would be using dead virus or an attenuated, or weakened, strain.

Although scientists often use the word inactivated loosely in describing vaccines, they generally use it for vaccines derived from killed virus. One example is the injected Salk polio vaccine. Scientists use the word attenuated to describe vaccines from live virus that has been weakened by growing it for long periods of time in test tubes. An example is the oral Sabin polio vaccine.

Mr. Tsang also declined to say what animals would be used for the SARS trials. Only after the animal trials are completed will a decision be made on whether to test the vaccine in people, he said.

The university issued a terse statement disclosing its plans for the trials after a local newspaper reported that animal tests would begin soon.

Doctors at Hong Kong University, Guangzhou Medical College and Fudan University Shanghai Medical College helped develop the vaccine.

A scientist knowledgeable about the research and who asked not to be identified said the announcement appeared premature because a vaccine had not been developed. A strain of the SARS virus has been inactivated with the aim of inserting it in the nostrils of mice to see how the immune system of mice responds to it.

"It is an inactivated virus but not yet a vaccine," the scientist said.

The plan for testing the strain coincided with further signs that the spread of the virus is slowing at least temporarily in Asia. Mainland China reported just eight new SARS cases today, along with three deaths. Hong Kong reported one new case and one death. Taiwan announced that it had 15 more cases and no deaths.