TUESDAY, April 28 (HealthDay News) -- As the death toll from swine flu in Mexico rises and new cases appear in the United States and elsewhere, it's easy to get caught up in a sense of mounting dread.
But experts in influenza and infectious disease say the exact level of danger from the virus is still far from certain.
"This is something of concern [but] I think we should hold back on calling it a real threat," said David Topham, co-director of the New York Influenza Center of Excellence, part of the University of Rochester Medical Center. "We always have to take these things seriously, but we have a very good system in place to respond."
Another expert agreed.
"The gravity of the situation will not be clear for a few more days till we find the extent of the cases and the number of countries involved and explain why we haven't had any deaths in the U.S," said Dr. Scott R. Lillibridge, a professor with the Texas A&M Health Science Center School of Rural Public Health in Houston and executive director of the university's National Center for Emergency Medical Preparedness and Response.
The real verdict on just how dangerous this outbreak might be will hinge at least partly on seeing how the disease spreads in the United States, along with expected announcements from the World Health Organization as to how many other countries are affected, the experts said.
So, as can happen in these situations, "the panic is a little bit undue," said Dr. Len Horovitz, a pulmonary specialist at Lenox Hill Hospital in New York City. "There are many facts that we don't know."
Looming large among the unknowns: Why is the virus causing more severe illness in Mexico than in the United States? Why have the only deaths reported been in Mexico? Why is mortality concentrated among young, healthy adults? Will the swine flu acquire more lethality as time goes on? And will the virus have a seasonality to it, like the "regular" flu?
Most of these questions have no solid answers -- at least not yet -- but there are some intriguing possibilities.
The death rate from the swine flu in Mexico (149 by late Monday) hovers at about 7 percent of the more than 1,900 so far thought to be infected -- a percentage that's alarmingly higher than the 1918 Spanish flu pandemic, which is thought to have killed about 2.5 percent of infected persons.
But this early in the outbreak, no one really knows the true extent of the Mexican outbreak, the experts noted, so that 7 percent figure might well be too high.
"We don't know the total number of people exposed," Topham said. "It could be many more people exposed, so we're only hearing about the ones who got really sick and died," he explained.
It's also not clear if all the deaths attributable to swine flu were actually caused by the virus.
"The strain in the U.S. seems to be the same as in Mexico, but in Mexico we've not had confirmation of all those hospitalizations or deaths," Lillibridge said. "We're comparing apples and oranges."
And to put things into perspective, even with the death toll in Mexico, the swine flu does not appear to be as dangerous as SARS (Severe Acute Respiratory Syndrome), which appeared seemingly out of nowhere in 2003.
"SARS had high human-to-human transmission, there was a high death rate and no treatment," Horovitz noted. With the swine flu outbreak, "we're not talking about anything like that," he said.