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A Flu Pandemic Is Expected to Happen Sooner or Later

'It's like predicting the Big One in California,' one scientist says. But 'we are overdue.'

November 02, 2005|Charles Piller | Times Staff Writer

No one knows whether the bird flu now migrating across the globe will cause a human pandemic, but researchers say it is inevitable that some flu virus eventually will.

"It's like predicting the Big One in California," said Dr. Arnold S. Monto, an epidemiologist at the University of Michigan and a former president of the American Epidemiological Society. "We are overdue for another pandemic. But we don't know when it will hit."

Unlike seasonal flu or more serious epidemics that can move through large regions, pandemics leap across the world, spreading through populations with little or no immunity.

In the last century, there have been three major flu pandemics, each of which originated with birds.

In 1918, the Spanish flu spread, killing 500,000 people in the United States and as many as 50 million worldwide -- more than all the battlefield deaths of World War I. In 1957, the Asian flu traveled across the world, killing 2 million people, including about 70,000 in the United States. The 1968 Hong Kong flu killed 1 million people, with 34,000 deaths in the U.S.

Multiple strains of flu virus circulate harmlessly in birds and humans. Small mutations cause a drift in the virus that produces seasonal flu.

Every so often, mutations create a significant shift in a strain that is especially unfamiliar to the human immune system.

Dr. Paul Offit, chief of infectious diseases at Children's Hospital of Philadelphia, said each of the various pandemic strains seemed to reemerge every 68 years.

"These viruses are always around ... always bubbling under the surface," Offit said. "You get to the point where the entire population is susceptible because nearly everyone around for the last pandemic has grown up and died," leaving few people with residual immunity.

The bird flu strain that has spread from Asia to Europe is known as H5N1. The first outbreak came in Hong Kong in 1997, forcing the government to order the eradication of chickens, ducks and geese.

Though the virus did not readily infect humans, there was a chance it eventually could mutate into a form that could be easily transmitted.

Recent research has shown that the H5N1 virus bears genetic similarities to the 1918 flu. And unlike many other flu viruses originating in birds, H5N1 can spread directly from poultry to people without passing through another species, such as pigs, increasing the risks.

"It's the most dangerous influenza virus that I've ever seen," said Richard G. Webster, a flu researcher at St. Jude Children's Research Hospital in Memphis, Tenn., who has conducted several key studies of H5N1.

Some experts downplay the likelihood of an H5N1 pandemic.

More than 120 million birds in Asia have died or been culled because of bird flu, but human infections have been relatively rare, about 120 cases since 2003.

Monto said genetic changes that made the virus easily transmissible could just as easily decrease its virulence.

"If it was easy for this to happen, it would have happened already," Monto said.

He estimated the chances of an H5N1 pandemic at no more than 5%.

But scientists agree that it is only a matter of time before some pandemic strain emerges.

In an age when travelers can spread disease across the world in a matter of hours, bolstering the nation's ability to manage the crisis is overdue, experts said.

"If a pandemic happens, it would be devastating," Monto said. "So we have an obligation to prepare."



Global threat

An avian flu pandemic could kill 2 million to 7.4 million people worldwide, according to the World Health Organization. Almost all of the 120 known human cases involve people who were in close contact with infected birds.


An influenza pandemic is an outbreak of disease that occurs when a new influenza A virus appears in the human population, causes serious illness and then spreads easily from person to person worldwide.

*--* Year Type Virus Global deaths U.S. deaths 1889 Russian flu H3N8 1 million N/A 1918 Spanish flu H1N1 Up to 50 million 500,000 1957 Asian flu H2N2 2 million 70,000 1968 Hong Kong flu H3N2 1 million 34,000


Sources: Centers for Disease Control and Prevention, World Health Organization, Associated Press

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