With relatively young, healthy people in Hong Kong dying of SARS, some infectious-disease experts are concerned that the respiratory disease could kill a broader range of victims than previously believed.
Hong Kong public health officials said severe acute respiratory syndrome claimed nine lives yesterday, including previously healthy women ages 32, 34 and 37 and a 45-year-old man. Over the weekend, five similar victims died.
The number of younger patients killed by the virus is a relative handful of the 154 fatalities and 3,235 sickened worldwide. Most who have died were elderly or had underlying health problems, as is typical of victims of viral infections such as influenza to West Nile.
But the newly reported cluster of deaths, in addition to a few early cases, has shown that SARS is capable of killing those not considered particularly vulnerable.
"What we can safely say is that this certainly does have the potential for causing death in younger people. We're seeing enough of it to be concerned," said Dr. Arnold Monto, a University of Michigan epidemiology professor.
Why SARS would kill younger people with no other apparent health problems remains a mystery. But infectious-disease experts and virologists said possibilities are numerous, from genetic predispositions to overly aggressive immune-system responses.
Possibly the simplest explanation is that doctors might not have caught the underlying health problems or personal habits that predisposed the Hong Kong victims to severe cases of SARS, said Dr. William Schaffner, chairman of the department of preventive medicine at Vanderbilt University in Tennessee and an expert on infectious diseases.
"Let's say smoking is a risk factor, or, for that matter, [intravenous] drug use," he said. "We're not sure about all the details yet."
Another possible reason for the deaths is that SARS is caused by a previously unrecognized coronavirus, which means no one of any age has developed immunity to it.
"If this is a unique, new virus, really unrelated to the other human coronaviruses and to other things that infect people, there's really no reason why the average person should have antibodies and be protected from it," said Dr. Arthur Reingold, head of epidemiology a the University of California, Berkeley's School of Public Health. "Most of us get our protection through getting infected with things."
The 1918-1919 Spanish influenza pandemic is a frequently noted example of the killing power packed by a new virus. That pandemic killed at least 20 million and affected a disproportionately high number of younger adults. The pandemic killed one out of every 100 U.S. residents between the ages of 20 and 40, about the same death rate as that for people older than 70, said Monto, the Michigan epidemiologist. No one has determined why.
In general, a simple formula can be used to predict whether someone will develop a disease, said Dr. Harold Standiford, medical director of infection control at the University of Maryland Medical Center. The formula is the amount of the invading organism in a person's body multiplied by its virulence, divided by how well that person can fight it off.
"Part of it can be how much virus you get to begin with, because basically when your immune system kicks in, it's sort of a race between the virus replicating in your body and your immune system trying to catch up and control it," said Gregory E. Glass, professor of microbiology and immunology at the Johns Hopkins University Bloomberg School of Public Health.
People who have high viral loads - measured by the amount of virus in the bloodstream - of HIV, for example, are more likely to develop severe cases of AIDS, and more quickly, than those with lower viral counts.
And people who eat large quantities of undercooked chicken at a barbecue are more likely to develop severe cases of food poisoning caused by salmonella bacteria, and more quickly, than those who eat less.
Viral load could explain why some people are believed to be "super spreaders" - seemingly more adept at spreading SARS than others.
When some patients cough or sneeze, they might spew droplets containing large amounts of the virus, a phenomenon similar to that of tuberculosis infections. Patients with TB-filled cavities in their lungs cough up far more germ-laden sputum than those with milder cases.
The younger, seemingly healthy people who succumbed to SARS yesterday in Hong Kong might simply have lost the race to beat back heavy loads of the virus.
But public health officials looking into the deaths also might want to investigate whether the victims had "an over-exuberant inflammatory response" in which the immune system ends up working so hard that it also damages healthy cells, said Stephen S. Morse, an emerging-diseases expert at the Columbia University Mailman School of Public Health.
Such a response wouldn't be without precedent.
"Much of the damage from hepatitis B is from immune response," said Dr. Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota's School of Public Health. In such cases, he said, the immune system "will attack cells that may be infected and result in collateral damage" to surrounding cells.
Genetic reasons are also possible.
"We know people differ in how they respond to general infections, be it cold or flu or measles or mumps or whatever. It's your genetic background," said Glass.
SARS - characterized by a fever higher than 100.4 degrees, body aches, a dry cough and, in severe cases, pneumonia - hasn't killed anyone in the United States.
Wire services contributed to this article.