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The World Health Organization's definition of what constitutes a pandemic is too broad, according to an article published this week.

Peter Gross, infectious disease specialist with the Hackensack University Medical Center in New Jersey, has penned an editorial for the British Medical Journal's Clinical Evidence in which he suggests that the WHO's loose definition could lead to false alarms.

The WHO changed their definition last year, Dr. Gross said. Previously, a "shift" in the virus would have to occur, meaning a new subtype of the virus would have to appear in order for a pandemic to be declared.

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A subtype is represented in the H and N numbers of a flu virus' name, such as H1N1. Another example is avian flu, which is categorized as H5N1 or, technically, as Influenza A/H5N1.

Now, with their recent change, Dr. Gross said the WHO has made the definition too vague. "They said a new animal or human-animal strain could qualify as a pandemic strain. That's too non-specific." He said neither shifts nor drifts, which are subtle changes in the virus' makeup, are mentioned specifically in the guidelines for declaring a pandemic.

As of this year, the WHO defines a pandemic as occurring "when an animal influenza virus to which most humans have no immunity acquires the ability to cause sustained chains of human-to-human transmission leading to community-wide outbreaks. Such a virus has the potential to spread worldwide, causing a pandemic."

Dr. Gross says the new definition could lead to false alarms and that "any minor change, any minor drift could be considered a pandemic."

He also pointed out that, even in a medical dictionary, the definitions for the terms "pandemic" and "epidemic" are not clear, with no guidelines set on the number of people who need to be affected to reach either status, and no specific mortality rate.

In their 2009 Pandemic Influenza Preparedness and Response document, the WHO defined a pandemic as simply "an epidemic on a global scale."

Dr. Gross said, "H1N1 has been around for most of this century, except for 20 years mid-century, from 1957 to 1976. Nobody knows where it went, but then it re-appeared in '76."

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He said that since 1977, H1N1 has been included in seasonal flu vaccines, along with H3N2.

However, Dr. Gross said the current strain of H1N1 has undergone a significant drift in its makeup, thereby making humans less resistant to it.

"A seasonal flu causes about 36,000 deaths," he said, referring to statistics in the United States. "The difference with what's going on right now is we have more morbidity and mortality among young children and young adults. Usually, it's the other end of the age-spectrum that's affected."

The WHO pandemic document includes statistics for the three major pandemics of the 20th century. The worst was the Spanish Flu of 1918-1919, which also targeted young adults and killed 2 to 3 per cent of those who contracted it, worldwide.

"Influenza is different from most other human viruses," he said. "When you get the measles vaccine, that lasts for a lifetime because the virus is genetically stable." Influenza, on the other hand, is not as stable because its genetic makeup is split into pieces. If one strain meets another, the pieces mix together. "When it comes out, who knows what it's going to look like?"

"We just need a better definition of all of this," Dr. Gross said. "It's really kind of amazing after all these years that it's not clearer."

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