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

By William Booth, Anne-Marie O'Connor and Joshua Partlow
Washington Post Foreign Service
Saturday, May 2, 2009

MEXICO CITY, May 1 -- Mexican health officials studying the new influenza virus said Friday they have found that its ability to spread from person to person may be fairly low, raising hopes that the extreme measures taken here -- the shutting down of all nonessential commerce and government -- can contain its spread.

In an obscure government building in the south of this city, dozens of experts in public health gathered in a "war room" to monitor on computer screens the spread of swine flu around the country. While it is far too early to answer with any certitude the most pressing questions -- how infectious and lethal is the virus? -- they offered some preliminary assessments.

Hugo López-Gatell, director of epidemiology and disease control for the Mexican Health Ministry, said one reason for higher mortality among Mexican patients is that they delay seeking help. "One of our biggest lessons was that people delay in getting to health centers," López-Gatell said. "Prompt attention diminished the deaths."

Steve Waterman, a team leader at the U.S. Centers for Disease Control and Prevention who is in Mexico to assist with the health crisis, said, "Maybe there's something about using a lot of drugs at home that may make things worse. Maybe Mexico City's high elevation . . . makes it more likely for you to have problems."

Waterman added, "They have excellent doctors and antiviral drugs, so they should be able to manage cases well." But some patients may be waiting until they are extremely sick before seeking medical help, he said. "If they come in late, there's often nothing you can do."

There have been 397 confirmed cases, and 16 of those patients have died. The number of confirmed cases is expected to rise dramatically, as Mexico has just put in place laboratories to test for the virus.

Asked to estimate the mortality rate of the outbreak in Mexico, Waterman said, "That's one of the major questions we're trying to answer."

"Mexico is in the earlier stage of transmission," he said.

Unknown Point of Origin

Scientists still do not know where the virus originated. In one major outbreak that tipped off scientists that something was amiss in Mexico, 616 people out of a population of 2,155 got sick in the town of La Gloria, which is surrounded by pig farms in Veracruz state. But Miguel Ángel Lezana, the director of the National Center for Epidemiology and Disease Control, said he doubts that pig farms caused the virus. There is only one confirmed case of swine flu in La Gloria, a 5-year-old boy who recovered.

"I don't think so. The farms are quite far," Lezana said. "Our agricultural authorities already made some analysis with the pigs there. They found nothing."

Said the Health Ministry's López-Gatell, "We don't know yet. It could have been in California, where there were the first known cases, or it could have been in any other part of the world, or in Mexico, of course."

López-Gatell said health officials noticed an increase of serious flu cases in the annual flu season, which runs from October to March. In the previous season, they had seen 4,000 serious cases of flu by March 2008. During this season, they had seen 4,000 such cases by December. Further outbreaks throughout the country brought an additional 3,000 cases by March -- a total of 7,000 for the season.

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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