ATLANTA — Why has the swine flu engulfing Mexico been deadly there but not in the United States?
Nearly all those who died in Mexico were 20 to 40 years old, and they died of severe pneumonia from a flulike illness believed caused by a unique swine flu virus.
The U.S. victims cover a wider age range, as young as 9 to older than 50. All of them either recovered or are recovering; at least two were hospitalized.
“So far we have been quite fortunate,” Dr. Anne Schuchat of the U.S. Centers for Disease Control and Prevention said Saturday, just hours before three new U.S. cases were confirmed.
Health experts worry about a flu that kills healthy young adults, a hallmark of the worst global flu epidemics. Deaths from most ordinary flu outbreaks occur among the very young and very old.
Why the two countries are experiencing the illness differently is puzzling public health experts. It may be that the bug only seems more deadly in Mexico.
And while experts believe Mexico is the epicenter of the outbreak, they’re not certain if new cases are occurring, or if the situation is getting worse. They also don’t know if another virus might be circulating in Mexico that could be compounding the problem.
A big question is, just how deadly is the virus in Mexico?
The seasonal flu tends to kill just a fraction of 1 percent of those infected.
In Mexico, about 70 deaths out of roughly 1,000 cases represents a fatality rate of about 7 percent. The Spanish flu pandemic of 1918-19, which killed an estimated 40 million worldwide, had a fatality rate of about 2.5 percent.
The Mexican rate sounds terrifying. But it’s possible that far more than 1,000 people have been infected with the virus and that many had few if any symptoms, said Dr. Michael Osterholm, a prominent pandemic expert at the University of Minnesota.
U.S. health officials echoed him.
“In Mexico, they were looking for severe diseases and they found some. They may not have been looking as widely for the milder cases,” said Schuchat of the CDC.
The U.S. health agency sent two investigators to Mexico on Saturday to help, she said.
Currently, even the counted illnesses are problematic. Only a fraction have been lab confirmed. Severe pneumonialike illness happens all the time, so it’s challenging to figure out which ones are really tied to the outbreak, U.S. health officials said.
“These numbers need to be confirmed,” said Dr. Richard Wenzel, the immediate past president of the International Society for Infectious Diseases.
Other ideas about the difference include:
• Genetic analysis of virus samples in the two countries is continuing. The CDC says tests results show the U.S. and Mexican viruses are essentially the same, but some experts have not ruled out the possibility that the virus is changing as it leaks across the border to the north.
• Perhaps nutrition levels are worse in some Mexican communities. Poor nutrition can degrade a person’s immune defenses, and make them more susceptible to illness.
• Air quality in Mexico City is considered terrible. That too may have affected patients confronted with a novel respiratory disease.
• Access to medical care has been an issue in Asia, where a rare bird flu, which does not spread easily from person to person, has killed more than 200 in the past several years. Maybe Mexican patients have also had trouble getting medical care or antiviral drugs, some have speculated, even though the government provides health care.
“The question of why the virus appears to be more virulent in Mexico is one that we are looking intensively into,” the CDC’s Schuchat said.