Wednesday, April 29, 2009

Putting Mexican Swine Flu In Perspective

April 29, 2009

Ninety-one years ago, the world was on the precipice of what was to become the most devastating influenza pandemic in recorded history. 

By the time it burned out, 20% of the world's population – including 28% of the US population – had been infected; an estimated 20 million to 100 million people died. The death rate was a staggering 2.5% (see Footnote). 

The US population in 1918 (excluding members of Armed Forces serving overseas) was approximately 103 million. This means that almost 29 million Americans were infected by the 1918 pandemic flu strain, and more than 720,000 of them died from their infection. The impact of the 1918 pandemic was so severe that the US population decreased by approximately 60,000 in 1918. In comparison, the country's population increased by 1.3 million people in both 1917 and 1919.

In contrast to the 1918 pandemic, seasonal influenza outbreaks in the years 1976-1999 were responsible for an average of 34,470 deaths annually. This toll represented 19.6 deaths per 100,000 population. The 1918 death toll of 720,000 translates to 699 deaths per 100,000. The 1918 pandemic strain – based on US statistics – was 35 times more lethal than the typical seasonal influenza strains that pop up each year.

How does the Mexican swine flu strain fit this picture?

Taken on the surface, the latest official World Health Organization statistics from Mexico seem frightening – 7 deaths out of just 26 confirmed human cases; a 27% lethality rate. But these numbers are deceiving. Mexico does not have the lab facilities to confirm swine flu cases. Its initial samples were sent to Canada's national reference lab, located in Winnipeg, for analysis. 

The Mexican government suspects that the current outbreak may be responsible for as many as 2,498 illnesses and 159 deaths. More than one-half of the suspected swine flu victims – 1,311 people – are still hospitalized. But these number don't account for the likelihood that many more Mexicans have suffered milder symptoms and either never consulted a doctor or were seen by a doctor but did not require hospital treatment. 

The story that is evolving outside of Mexico may eventually give us a better picture of the lethality of this latest influenza strain. As of this morning, cases of Mexican swine flu have been confirmed or classified as "probable" in a number of countries:

According to a report released this morning by the Texas Department of State Health Services, the US death involved a 22-month old child from Mexico City who was visiting family in Brownsville, TX. The child fell ill 4 days after leaving Mexico City with his parents, was hospitalized in Brownsville, and then transferred to a Houston-area hospital for treatment. The child, who had other underlying health issues, passed away while in the Houston-area hospital. No one who was in close contact with him has developed symptoms of influenza.

The potential of this influenza strain to cause illness should not be underestimated. But neither should we panic. Certainly, canceling or postponing travel to severely affected areas is appropriate. Avoiding large crowds in confined areas is appropriate. Staying home when ill is appropriate. Paying careful attention to personal hygiene – frequent hand-washing, covering nose and mouth with a tissue when sneezing or coughing, avoiding touching one's eyes or nose with one's hands – is appropriate.

Wearing a face mask is of limited value. Face masks, if fitted correctly, can stop aerosols and large dust particles on which the influenza virus may have hitched a ride. But a face mask is not able to filter virus particles out of the air and may give its wearer a false sense of security.

What about the risk of eating pork?

The influenza virus is a respiratory virus. People become infected by inhaling it. Countries – Russia, Ukraine, China, and others – that have placed an embargo on Mexican and US pork are acting based on politics, not science. And Russia's announced ban on US beef and poultry because of the current outbreak is completely out of line.

USDA announced yesterday that the Mexican swine flu virus has not been detected in any swine herds in the United States. Indeed, not one infected swine herd has been found in any areas where human cases have been reported. Secretary of Agriculture Tom Vilsack said, in part,

"I want to reiterate that U.S. pork is safe. While we in the U.S. are continuing to monitor for new cases of H1N1 flu, the American food supply is safe.

There is no evidence or reports that U.S. swine have been infected with this virus. USDA is reminding its trading partners that U.S. pork and pork products are safe and there is no basis for restricting imports of commercially produced U.S. pork and pork products.

This is not an animal health or food safety issue. This discovery of the H1N1 flu virus is in humans. Any trade restrictions would be inconsistent with World Organization for Animal Health (OIE) guidelines."

What's next?

More countries will confirm cases. The number of new confirmed cases worldwide will increase, peak, and decline. There will, sadly, be more deaths. It's too soon to speculate on the ease with which the virus spreads person-to-person, or how lethal it will prove to be. 

The virus might remain in circulation, might mutate further, or it may simply disappear into the background noise of other influenza strains that have popped up and then faded away. 

And, sooner or later, another new influenza strain will appear to test our pandemic preparedness.

Footnote: The statistics quoted in the introductory paragraphs are taken from Gina Kolata's excellent book, "FLU: The Story of the Great Influenza Pandemic of 1918 and the Search for the Virus That Caused It", published in 1999 by Farrar, Straus and Giroux.

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