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H5N1: The Next Pandemic?
A highly virulent and deadly new influenza virus strain is emerging in Southeast Asia that is of great concern to health administrators and infectious disease specialists. The new virus is called H5N1 avian influenza virus type A. Many infectious disease experts think we are on the verge of a major worldwide influenza pandemic of similar severity to the 1918 Spanish Flu.
What Is a Pandemic?
An epidemic is defined as an infectious illness that spreads so quickly that the number of new cases rises in an exponential manner rather than just increasing linearly. This means that the number of new cases doesn't just go up by ones or twos each day, but doubles every few days. A pandemic is an outbreak of disease that affects every continent rather than just one geographic area.
One of the most important reasons for influenza's success in spreading epidemics is its infectivity, or how easily it is transmitted from one person to another. Infectious agents that can cause illness after a small exposure are more contagious than ones that require a larger exposure. Infectivity is increased when infection can be passed between people without any direct contact.
The most common way for flu to be transmitted is by breathing contaminated air. It can also be transmitted by direct contact with an infected person, shaking a hand or touching something that the sick person previously touched. Under the right conditions, flu can remain infectious for days outside of the human body, living on surfaces like counter tops or doorknobs.
The virus can only infect someone if that person is susceptible or vulnerable to it. Unfortunately, virtually 100 percent of the human population is susceptible to a new strain of virus, like avian flu, because we don't have any immunity to it.
Influenza causes pandemics because it scores so highly in all these causes of infectivity. These characteristics of influenza help explain why this organism can quickly spread from one region of the globe to another.
During flu pandemics, a higher than usual percentage of the population becomes infected and more people die than during the usual annual flu season. Pandemics occur because a new influenza virus makes its way from birds or swine to humans, resulting in a strain for which we have very little immunity.
Major and Minor Pandemics
There are major pandemics and minor ones. Minor ones are more common and much less severe than major ones, but still a lot worse than routine flu outbreaks we experience each winter. All pandemics infect many times more people than happens with the seasonal flu but during major pandemics, like the Spanish Flu
Mortality Rate from Infectious Disease in the United States
(Per 100,000 People)
Note the spike in deaths during the 1918-1919 Spanish Flu epidemic of 1918, the death rates soar into the tens of millions or even higher.
Armstrong et al. JAMA 1999; 281:61–66
On average there are two minor pandemics for every one major pandemic. The minor pandemics are associated with lower clinical attack and case fatality rates than in major pandemics. For instance, the 1957 pandemic was associated with three times as many deaths than seen for seasonal flu but during the 1968 flu pandemic, there were only a few more deaths than would be expected. It has now been thirty-seven years since the last flu pandemic, which suggests we may be due for another one soon.
Pandemic Death Tolls
During the 1918-1919 pandemic, five to ten times as many people as usual became severely ill with flu, and many millions died from their infection. The percentage of the population that becomes ill with flu symptoms is known as the clinical attack rate. The medical term for the percentage of people who become ill and die is the case fatality rate. During the usual winter flu season, the case fatality rate hovers around 0.2 percent to 0.35 percent. During minor pandemics, this rate increases up to 3 to 4 times; during a major pandemic the rate increases by 10 to 50 times!
Pandemics Can Be Predicted
One of the things that surprises many people is that influenza pandemics are regular events with an almost predictable periodicity of three per century. In fact, over the last four hundred years there have been twelve recorded flu pandemics. Every hundred years or so, a major pandemic occurs that dwarfs everything else by comparison.
Most flu experts predict that it is only a matter of time before the bird flu virus mutates to become transmissible between people, so that is not the burning question. The question is: how soon?
According to the World Health Organization guidelines for pandemics, as of September 2005 we are in Phase 3. This places us in the Pandemic Alert Period, just one step away from human-to-human spread that will be followed by a worldwide pandemic. Even during the relatively primitive travel conditions existing in 1918, it took only six weeks for epidemic influenza to spread from the United States to Europe and Africa. Imagine how fast the next pandemic virus will move across the globe given the many thousands of passengers traveling internationally by air every day. Taking this into account, the British Government's Health Protection Agency predicts in their Influenza Pandemic Contingency Plan that once the first case of pandemic flu reaches Hong Kong it will take only two to four weeks to arrive in the United Kingdom.
My estimate is that when the avian flu crosses over to humans, there is a one in three chance of a major pandemic and a two out of three chance of a minor one. The most likely time for this to happen in the Northern Hemisphere is during the annual flu season from December 2005 through March 2006. This estimate is based on several factors. Most virologists agree that for H5N1 to achieve efficient human-to-human transmission it will require more human flu RNA; the time when this is most likely to happen is during the annual flu season which runs from December through March. The next most likely time would be December 2006 through April 2007.
Influenza pandemics occur in waves. The 1918 Spanish Flu (H1N1) was associated with three waves, while the 1957 Asian Flu (H2N2) and 1968 Hong Kong Flu (H3N2) had two distinct waves each. The reason for this wave behavior is not known, but some speculate it's due to a change in the season of the year. The timing of a wave may also be related to a genetic change or mutation in the new strain of influenza virus. In past pandemics, the time between two waves was three to nine months. One point to keep in mind about pandemic waves is that the second wave can be much more severe than the first or third wave of the series. During the 1918 pandemic, the deadly second wave was responsible for more than 90 percent of the deaths for the entire pandemic.
While the typical flu season predictably occurs from November through March, during pandemics, flu can vary from this script. The first wave of the 1918 flu spanned January through March. That flu was very severe by usual standards but the second wave, beginning six months later in September, was the most fatal. The third wave occurred during the following winter/spring and was the mildest of all. Pandemics end, not because of a miracle cure or a magic-bullet vaccine, but simply because all or most susceptible persons within the population have contracted the infection and developed immunity to it, or they died.
Pandemics Strike the Healthiest, Too
During pandemics, a major difference compared with seasonal flu is that there is a high death rate among healthy twenty- to thirty-year-old adults. This is in contrast with the seasonal flu that strikes the very old, the young, and the infirm the hardest. Of course, the usual victims of seasonal flu are not spared during pandemics. On the contrary, death rates are much higher for every age and risk group during pandemics compared with seasonal flu. The point here is that the twenty-to-thirty- year-old group, usually immune to the ravages of seasonal flu, experiences very high death rates. Ironically, one possible explanation for this pandemic observation may relate to the increased health and vigor of this group's immune system.
©2007. Grattan Woodson . All rights reserved. Reprinted from The Bird Flu Preparedness Planner . No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, without the written permission of the publisher. Publisher: Health Communications, Inc., 3201 SW 15th Street , Deerfield Beach , FL 33442.