Mexico Scientists Say Flu's Ability to Spread Is 'Fairly Low'
By Joshua Partlow and William Booth
Washington Post Foreign Service
Friday, May 1, 2009 4:06 PM

MEXICO CITY, May 1 -- Mexican scientists studying the influenza virus here have found that its ability to spread from person to person is "fairly low" and no more infectious than normal seasonal flu, giving them hope that they can better contain its spread.

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.

With 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 is between 1.5 and 3.

"According to the preliminary models, the reproductive number that we have in the Mexico City metropolitan area is 1.5," said Miguel Angel Lezana, director of the National Center of Epidemiology and Infectious Disease.

"It's a number fairly low, and that's good news," Lezana told The Washington Post in an interview at his Mexico City office late Thursday.

"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."

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

Scientists, for example, are still debating how infectious the severe acute respiratory syndrome (SARS) outbreak of 2003 was, which has generally been assigned a basic reproductive number of 3. The most recent estimate of the infectiousness of 1918 influenza pandemic virus range between 1.8 and 2.0.

Reaction to the early findings in Mexico was cautiously optimistic.

"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, professor of epidemiology at Harvard School of Public Health who is now assisting the U.S. Centers for Disease Control and Prevention in battling the outbreak. "I don't know the basis for the estimates so can't comment except to say that 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."

Neal L. Cohen, former New York City health commissioner, said "that if the number is accurate, it might give us some comfort." Cohen, a lecturer at Hunter College School of Urban Public Health, said, "If the number remains 1.5 and the virus does not undergo further mutations, it's hopeful."

Other researchers agreed. "It's certainly low," said Thomas G. Ksiazek, director of the National Biodefense Training Center at the University of Texas Medical Branch at Galveston. "I don't have the data so its impossible for me to say that number and if how they derived it is accurate."

In Mexico, health officials on Friday said that 18 people have died because of confirmed cases of the new virus. There are another 326 confirmed cases of sick patients.

Lezana said, "The number moves. What we expect to see is this number will be lower in the next few days. The whole measures that are taking place here in Mexico City and in other parts of the country are precisely intended to stop the spread, introducing some barriers, by using some protective equipment, but most importantly to introduce distance between people. Those measures are intended precisely to lower that reproductive number. Hopefully if operations are effective we'll see that number decreasing in the next few days."

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http://www.washingtonpost.com/wp-dyn/content/article/2009/05/01/AR2...