Web Zap*Germs Archive

December 01, 2005

Avian-Flu Human Pandemic Chance: Less Than 50%, Says Infectious Disease Expert

Writing in the Richmond Times-Dispatch commentary section this past weekend, Dr. Richard Wenzel (chairman of Virginia Commonwealth University's Department of Internal Medicine and president-elect of the International Society for Infectious Diseases) discusses the origins of influenza and the current avian-flu danger. Wenzel says that in the nine years since the H5N1 flu appeared, it hasn't experienced the type of mutation that would make it deadly to humans, and he puts the chance that it will mutate in this way at less than 50%.

Some important points, emphasis mine:
From the perspective of the virus, the flu has two strategies to adapt in people and survive:

(1)It can have genetic mutations without subsequent proofreading in its assembly. The 1918 flu used this strategy.

(2)It can have "viral sex," essentially mixing its eight genes with the eight genes of a second, co-infecting flu virus. The reassortment can create 256 offspring varieties and usually takes place in a pig. This mechanism helps explain why epidemics originate in the Far East where birds, pigs, and people live in close contiguity. Numerous bird-pig reassortment viruses have been responsible for all human flu diseases in the U.S. after 1918.

Of the three varieties of influenza (A, B, or C), only influenza A causes global pandemics. "A" viruses in turn are classified on the basis of two surface proteins, H and N. The H protein is responsible for viral docking onto respiratory cells, thus initiating infection. The N protein allows newly minted viruses to escape from the human cells after assembly, thus freeing them to infect other respiratory cells.

The new avian influenza epidemic in Asia began in 1996 when a flu virus in geese (H5N3) infected quails and chickens with a second bird influenza virus containing an N1. After reassortment in birds, a new offspring emerged -- H5N1 -- and by 1997 people in China were reported ill with it. Direct bird-to-man transmission has also occurred in Vietnam, Cambodia, Thailand, and Indonesia. However, no sustained person-to-person spread has occurred so far.

Is a deadly pandemic of avian H5N1 influenza inevitable? The answer is no. I would place the odds at less than 50 percent. The likely viral strategies needed to create a person-to-person spreading virus include one or more of the following: an H surface protein mutation allowing it to adapt better to human airway cells; one or more mutations in the genes controlling viral reproduction rates; a co-infection with a human strain followed by viral sex (reassortment) allowing enhanced transmission in people. While all are possible, none has occurred since H5N1 emerged nine years ago.

What should people do now? In general the standard influenza vaccine is safe, usually better than 50 percent protective, and in preventing influenza infections and deaths would prevent reassortment should an avian strain such as H5N1 circulate. Second, influenza infections are transmitted by respiratory droplets and possibly by hand-to-hand after contamination with respiratory secretions. All individuals should cover their mouths before coughing and sneezing, and handwashing or cleaning with alcohol should be practiced assiduously.

What if avian flu adapts to people and causes a pandemic? First, new vaccines that induce antibodies in volunteers to bird flu have been identified and will begin to be produced in the next six to 12 months. In the meantime, two currently available drugs that interfere with the N protein release of virus from cells look effective in the test tube and are being stockpiled. Third, it may be shown that handwashing plus the use of a mask in crowds may be better than handwashing alone as it was with SARS in China. If so, the prudent use of a mask might prove helpful in a pandemic.


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