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Mexican Officials Say Flu's Ability to Spread May Be Low

Governments around the world are launching medical and clean up operations to protect citizens against swine flu infections.

In addition, he said: "There were many grave cases among young people around the country, in Baja California Norte and San Luis Potosi. We said, 'What's happening?'

Scientists characterize the infectiousness of a pathogen such as a new influenza virus by assigning it a "basic reproductive number," which is a measure of how many secondary cases of flu a typical patient will cause in a population with no immunity to the pathogen.

For measles, which is highly contagious, the basic reproductive number is above 15; for smallpox, it is above 5. For ordinary influenza, the basic reproductive number ranges from 1.5 to 3.0.

"According to the preliminary models, the reproductive number that we have in the Mexico City metropolitan area is 1.5," Lezana said in an interview. "It's a number fairly low, and that's good news."

"So looking at this number, the main point is that you have a great opportunity to stop the spread of the virus," Lezana said. "So yes, there is this problem with the spread within the family, but you have a good opportunity to stop the spread of the virus outside the family."

Shifting Estimates

The Mexican epidemiologists caution that their work is preliminary and that their understanding of the virus and its infectiousness may change.

Scientists, for example, are still debating the infectiousness of the virus that caused severe acute respiratory syndrome, or SARS, which has generally been assigned a basic reproductive number of 3. The most recent estimates of the infectiousness of the 1918 influenza pandemic virus range from 1.8 to 2.0.

"I think it's very early, and any number will be uncertain. And if it is low, that is good news but not a reason to expend less effort on control. Rather the contrary: The lower the number, the more readily control measures can reduce its spread," said Marc Lipsitch, a professor of epidemiology at the Harvard School of Public Health who is assisting the CDC in battling the outbreak. He added: "At this stage, any estimate must be preliminary, and small differences can make a big difference -- say, 1.5 is much easier to deal with than 2.0."

Lezana said, "The number moves. What we expect to see is this number will be lower in the next few days."


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