By Rob Stein
Washington Post Staff Writer
Saturday, April 26, 2003; Page A18

Different strains of the virus that causes severe acute respiratory syndrome may be circulating around the world, which could explain why the disease seems more dangerous in some places than others, scientists say.

Researchers have deciphered the entire genetic make-ups of more than a dozen samples of SARS virus, and preliminary analyses indicate there is a spectrum of genetic variation.

Although it is too early to draw conclusions from those variations, whether any affect how easily the virus spreads, how sick it makes people or how easily the immune system recognizes it, that spectrum could explain why SARS has appeared to be more transmissible and deadly in places such as Hong Kong and Toronto.

"Scientifically, it would not be at all surprising," said Robert G. Webster, a virologist from St. Jude Children's Research Hospital in Memphis, who just returned from Hong Kong. "The chances are very good that the virus that got carried into Toronto and Hong Kong were different."

Although U.S. health officials have been investigating more than 200 possible SARS cases in dozens of states, there have been no deaths in this country, and the disease has appeared to be much less serious here. One possibility is that only milder strains of the virus have arrived with infected travelers.

"Why are there these differences between what we've seen in Canada and the United States? In the United States, so far it seems to have been pretty wimpy," Webster said. "You can't tell me there's that much difference in the ability of the two countries to respond. I suspect there are different strains out there."

If there are different varieties of the virus, they could either be the result of different strains jumping from animals to humans at the same time, or of mutations that have occurred since the disease emerged in southern China in November.

SARS is caused by a previously unknown version of coronavirus. Coronaviruses are so-called RNA viruses, which are especially prone to mutations because their reproductive process has fewer safeguards. "It only takes one mutation in the critical area," said Webster, noting that just a single change in the genetic make-up of the flu virus can turn it from a relatively mild pathogen into a killer. "I would not be at all surprised if there were variants."

Klaus Stohr, the World Health Organization's top SARS scientist, said different varieties of the virus could explain the disparities in the disease. But Stohr cautioned that the only way to prove that a genetic variation is affecting how the virus behaves would be by studying large numbers of patients to see whether different strains are associated with different intensity of spread or disease. "For this coronavirus, we have no clue whether one amino acid change is going to make a big difference or what portion of the virus has to change," Stohr said. "It's much too early."

Stohr added that viruses often do evolve, especially if they recently moved from an animal to humans. "The virus is going to evolve. It's going to be subject to selective pressure," he said. "Certain substrains could be favored by this evolution." But the virus could easily evolve in ways that make it less deadly, or perhaps more easily transmitted but less likely to cause death, he said.

In evolutionary terms, it would be to the virus's advantage to be easily spread but not very deadly. That way an infected person would have more time to infect others, keeping the virus alive.

Julie L. Gerberding, director of the federal Centers for Disease Control and Prevention in Atlanta, spoke to what she called key questions. "I'd love to know why some patients are sicker than others and why in some populations there seems to be a higher attack rate of the pneumonia than in others," she said.

To try to answer these questions, WHO plans to establish a central repository of genetic information about the SARS virus so methodical analyses can be made, Stohr said.

In the meantime, public health officials will have to act based on the limited information available, Stohr said. "We are building a ship while we are sailing it. We have to make important public health decisions on what we know now."