WASHINGTON — More than a month into the swine flu outbreak that's now affected 13 countries, medical experts are wondering aloud whether the contagious disease will ever become the pandemic that everyone fears.
With at least 143 infections now confirmed in 23 states, the H1N1 virus continues to spread via person-to-person transmission.
The overwhelming majority of new cases, however, have been mild and haven't required hospitalization. Only one death, of a Mexican toddler, has occurred on U.S. soil.
As the disease migrates farther from its Mexican origins, where it's thought to have killed more than 150 people and sickened thousands of others, it hasn't yet packed the fatal punch that the world is bracing for.
That could change at any time because flu viruses are unpredictable and can mutate into a more dangerous strain in a short period of time.
The multitude of evidence, thus far, however, suggests that the swine flu virus won't follow the path of the 1918 flu pandemic that killed millions worldwide.
In fact, most experts still agree that President Barack Obama had it right when he said the outbreak is a cause for concern, but not alarm.
"I've said from the very beginning that I thought this might not play out so severely," said Matthew Boulton, an epidemiology professor at the University of Michigan School of Public Health.
In the aftermath of the anthrax attacks in 2001 and the more recent avian flu scares, Boulton said the nation's public health and security infrastructure "might be primed a bit for overresponse," to potential health threats.
For experts to compare the current outbreak to the 1918 pandemic is "pretty irresponsible," at this stage, Boulton said.
Boulton said it's "highly improbable" that the U.S. swine flu death toll will even approach the estimated 36,000 Americans who die each year of seasonal flu.
Lee Harrison, a professor of epidemiology and medicine at the University of Pittsburgh, agreed that preliminary data suggests the outbreak won't become a world pandemic, but he cautioned that the 1918 outbreak began as a mild form of influenza.
"It wasn't until it came back the following flu season that you really saw a real devastating pandemic in terms of death," Harrison said of the 1918 influenza. The second and third waves of the 1918 outbreak killed about 50 million people worldwide.
Harrison said it would take more work and time to determine the mortality rate from the current outbreak, but that "it does appear to be low and it doesn't appear to be in range with the 1918 pandemic. But again, it's a rapidly evolving situation and in 1918, it was the second wave that was particularly nasty."
Researchers at Northwestern and Indiana Universities have developed separate computer models that both estimate — in a worst-case scenario — that some 1,700 new swine flu infections will be confirmed in the U.S. over the next month.
Whether the current swine flu scare mirrors the 1976 swine flu outbreak at Fort Dix, N.J., remains unclear. More than one-quarter of the population was vaccinated in the 1976 swine flu outbreak, although fewer than 300 confirmed cases were identified and only one death occurred before it petered out.
It was early reports of Mexican fatalities that prompted concern about the current virus. But those deaths may only represent a very small percent of people who were actually infected with the disease, Boulton said.
Many more infections in Mexico may have gone unreported because patients had only mild symptoms that required no medical attention. Higher poverty rates and poor access to medical care may also have inflated the death toll, Boulton said.
Dr. Andrew Potter, the director of the Vaccine and Infectious Disease Organization at the University of Saskatchewan in Canada, said, however, that there's enough about the new virus to warrant preparation for the worst, particularly its propensity to attack young, healthy adults.
"Now who knows whether that's simply a matter of them being the first ones exposed in that first wave, who knows? But that's what's giving people at the World Health Organization and public health officials around the world reason to think twice about this," Potter said.
Seasonal influenzas kill mostly infants, the elderly and people with immune deficiencies. Potter said it will take another week or so to really get a handle on how serious the swine flu problem really is.
"You've got to be prudent because it very clearly is a threat. No question about it," Potter said. "Most of us are waiting for more data to give us more direction as to which way this thing is going to go."
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