MONDAY, May 11, 2009 (HealthDay News) -- The swine flu epidemic does have pandemic potential and is likely to be comparable to other 20th century pandemics, at least in terms of its spread, a new analysis concludes.
The report also suggests that the true number of -- largely unreported --swine flu infections in Mexico, the outbreak's epicenter, possibly had already reached 32,000 by the end of April. The World Health Organization's official tally for Mexico currently stands at 1,626 confirmed cases.
The situation could be similar in the United States. During a Monday afternoon news conference, Dr. Anne Schuchat, interim deputy director for science and public health programs at the U.S. Centers for Disease Control and Prevention, said that the 2,618 confirmed cases in the United States are likely just the tip of the iceberg. Many people who become ill don't seek medical attention and are never tested for this strain of flu, so "the numbers we are reporting are a minority of the actual infections that are occurring in the country," she said.
The authors of the study, released online Monday in the journal Science, estimated that the current H1N1 swine flu outbreak is likely to wreak less havoc than the 1918 Spanish flu pandemic that killed more than 500,000 Americans.
Instead, it may prove similar to the much less lethal 1957 pandemic of Asian flu, which killed about 70,000 people, according to U.S. government statistics.
For reference, about 36,000 people die in a typical flu season.
However, the authors are basing their conclusions mainly on infection rates, not on severity of the disease or number of deaths, added Dr. Ghinwa Dumyati, an associate professor of medicine, division of infectious diseases at the University of Rochester Medical Center.
"Not every pandemic is severe, and that's the issue," she said. "Severity does not depend on the number of people infected but on the fatality ratio."
Another expert agreed. "The most important unknown is the clinical severity of the illness, although the accumulating evidence is that the severity is no worse than that of the seasonal flu," said Dr. Christopher Crnich, an assistant professor of medicine in the division of infectious disease at the University of Wisconsin School of Medicine and Public Health. However, "no mathematical model is going to be able to predict [severity]," Crnich added.
So far, the swine flu does not seem to have the level of sustained human-to-human transmission of a "true" pandemic, Dumyati added.
The World Health Organization (WHO) recently raised its global pandemic alert to phase 5 -- just shy of the highest level possible, which is phase 6, signifying a global pandemic.
According to the WHO Rapid Pandemic Assessment Collaboration, which produced the new Science paper, the new analysis supports the phase 5 decision, and indicates a certain level of person-to-person spread.
Another expert agreed.
"The information we have is still incomplete, but this can help lay out a scientific basis for political decisions," said Dennis M. Perrotta, associate director for emerging infectious diseases at the National Center for Emergency Medical Preparedness and Response. "The characteristics of the ongoing epidemic they examined, mostly in Mexico, clearly has sustained human-to-human transmission and we know this is also true in the U.S. It says that, yes, it was an appropriate response on the part of WHO to move the alert to level 5."
But the hallmark of this outbreak has been the large number of unknowns -- including how virulent the virus is and how it spreads -- and that is true of this analysis as well, the authors warned.
Nevertheless, such information is necessary to inform public health decisions, experts say.
Confirmed cases of H1N1 swine flu in the United States climbed to more than 2,600 across 43 states by Monday, including three deaths, and the United States now surpasses Mexico as the country most affected by the outbreak, according to WHO figures. The vast majority of cases remain mild, however.
So far, U.S. deaths linked to swine flu occurred in individuals with underlying health problems.
On Monday, the WHO was reporting 4,694 confirmed cases of swine flu in 30 countries, with Canada, Spain and the United Kingdom having the most cases outside of the United States and Mexico.
For every confirmed case, however, there are probably hundreds of unconfirmed cases, noted Dr. Gordon Dickinson, chief of infectious diseases at the University of Miami Miller School of Medicine and the Miami VA.
Three influenza pandemics swept across the globe in the 20th century: the so-called Spanish flu epidemic of 1918, the "Asian" flu in 1957 and "Hong Kong" flu in 1968. Pandemics are labeled as such whenever a new flu virus appears and has sustained human-to-human transmission.
Based on an analysis of confirmed swine flu cases in Mexico and its international spread, the WHO researchers estimate that 23,000 individuals (possibly up to 32,000) had been infected in Mexico alone by late April, with a death rate of about 0.4 percent.
That's much less lethal than the 1918 pandemic, which killed 2.5 percent of people infected.
But it may be too early to breathe a sigh of relief, experts said. The swine flu outbreak could be dying down or it could simply gearing up for a deadlier resurgence in the fall and winter.
"It's too early to say," Dumyati said. "At this point, it doesn't look like it has sustained transmission, but you don't know, in the winter, what will happen."
"People need to stay tuned and see what's going on, and the medical community needs to work fast and hopefully, a vaccine will be prepared," Dickinson added. "What bothers me is that, in 1918, the influenza emerged in the spring then sort of melted away during the summer. Then it came back as a very lethal process. This gives one pause as we see this current H1N1 epidemic spreading . . . The cases continue to emerge. It just hasn't hit with a major force yet."
U.S. public health departments are so overwhelmed, Dickinson said, that they are asking facilities not to send samples for testing unless there is a cluster of outbreaks or a particular clinical need.
"They can't keep up," he said.
Visit the World Health Organization for more on the current H1N1 swine flu outbreak.