Virulence of Swine Flu Is Not Yet Known
Saturday, May 2, 2009
So is this new swine flu outbreak the next great plague, or just a global spasm of paranoia?
Are we seeing a pandemic or a panic?
The pathogen that has seized the world's attention has an official name (swine-origin influenza A H1N1), an acronym (S-OIV), a nickname (swine flu) and an apparent birthplace (Mexico). But the essential nature of the pathogen, its personality, its virulence, remain matters of frenetic investigation. Like all influenza viruses, it is mutating capriciously and, thus, is not a static and predictable public health threat but an evolving one.
The bug has gone global, having shown up in Asia yesterday with the first reported case in Hong Kong. It also popped up in Denmark, as well as in eight new U.S. states.
But there has been some flu-scare backlash, with some officials questioning whether schools are too quick to close their doors at the first hint of the virus.
The World Health Organization directly addressed the pandemic-versus-panic issue yesterday by cautioning the public against leaping to any conclusions about the virulence of the virus. It has yet to show lethality outside Mexico (the one person to die in the United States was a toddler who traveled from Mexico to Texas), though that doesn't mean it will remain a mild pathogen in the weeks and months to come, officials said.
Influenza is a simple virus, with just eight genes, but it makes poor copies of itself, leading to constant mutation. Most of those mutations are dead ends, but, given enough chances, the virus can become more infectious or more lethal. Although the United States is past its flu season, the Southern Hemisphere, where the virus has spread, is entering the cold months when influenza can become explosive.
Some positive news surfaced yesterday: Mexican scientists said the contagiousness of the swine flu is no greater than that of the seasonal flu that circulates every year. And a preliminary genetic analysis hasn't turned up any of the markers that scientists associate with the virulence of the 1918 "Spanish" influenza virus, said Nancy Cox, head of the flu lab of the Centers for Disease Control and Prevention.
The 1918-19 pandemic has cast a long shadow over today's health emergency. That virus circled the world, eventually infecting nearly everyone and killing at least 50 million people.
Jeffery Taubenberger, the National Institutes of Health researcher who reconstructed the 1918 influenza virus, said he is growing the new swine flu virus in his lab.
"We're very early on in figuring out what makes this virus tick. I am loath to make predictions about what an influenza virus that mutates so rapidly will do," he said. But he believes it will spread across the planet: "My prediction is that this strain will continue to spread, and it is very likely to become a pandemic virus, if it's not already a pandemic now. That does not mean that this has to be a very severe pandemic like 1918."
Michael T. Osterholm, an epidemiologist at the University of Minnesota, said the situation is analogous to forecasting a hurricane when meteorologists know only that there is a high-low pressure gradient in the Atlantic. "Everyone in one week wants an answer as to what it will do. Anyone who gives you an answer right now, do not listen to them about anything else because you cannot trust them," Osterholm said.
WHO spokesman Gregory Hartl noted yesterday that the public may misunderstand the word "pandemic." The term refers to where an illness spreads, not its severity.
A major unknown is the swine flu virus's "case-fatality rate" -- the small fraction of infected people who die. For the 1918 influenza, it was 2 to 2.5 percent for the United States as a whole, but in military camps and on troop ships, the rate was a brutal 7 to 10 percent, and in some Inuit villages, it soared to 70 percent.
The other two flu pandemics of the 20th century, however, were far milder. The Asian influenza of 1957-58 had a fatality rate of 0.2-0.5, and the rate during the Hong Kong influenza of 1968-69 was even lower, about 0.1 percent, close to what it is for seasonal flu.
The case-fatality rate of the swine flu will become certain only when epidemiologists are able to track its behavior from the moment it arrives in a population -- a difficult task under the best circumstances, which the current circumstances in Mexico aren't. Physicians there first suspected something strange when a small number of young adults showed up in the hospital with severe pneumonia.
The question is how many other people contracted influenza but never got very sick. Researchers must draw blood from a sample of people in affected towns and cities to estimate how many people were infected and never knew it.
The early signs from the United States and a few European countries where the strain is spreading suggest it is not unusually dangerous, as there have been few deaths so far. If that continues to be true, then it may help explain the mysteriously high mortality in Mexico. It may be that Mexico already has had hundreds of thousands, and possibly millions, of cases -- all but the most serious hidden in the "noise" of background illness in a crowded population.
The fact that most people infected in other countries had recently been to Mexico -- or were in direct contact with someone who had been -- is indirect evidence that the country may have been experiencing a silent epidemic for months.
Regardless of how dangerous it proves to be, the new swine flu virus is almost certain to eventually infect every continent and country, although that may take years. Studies in the 1930s found that 97 percent of people born before 1920 had antibodies to the Spanish influenza virus. That's evidence that virtually everyone alive in the three years it circulated -- 1918, 1919 and 1920 -- was at one point infected, even if they didn't know it.
A similar fate awaits any population exposed long enough to a new flu strain to which it has no immunity, experts believe.