Dr. Robert DeBell is a senior member of the CBMTS Executive Committee and a Senior Member of the Independent ASA Board of Advisors

How Dangerous is Influenza?

by Robert M. DeBell, Ph.D.

          Although too often the general public does not consider influenza a serious disease, both the research and medical communities realize the influenza virus is capable of being a serious killer, especially for the young and old. In the widely recognized pandemic of 1918, about 20 million people (n.b., 500,000 deaths in the U.S.) died from the infection(1); however, some estimate the death toll to have been between 50 and 100 million.(2) This was at a time when transportation was far more limited than at present and when the world population was less than two billion, approximately a third of the population now. In balance, however, present day medicine and available pharmaceuticals are far more effective than during the early 20th century.
          Although the 1918 occurrence may be most remarkable, experts predict that another serious influenza pandemic will occur eventually. Other less lethal pandemics have occurred in 1957 and 1968. The Asian flu outbreak in 1968 resulted in 50,000 U.S. deaths.(3) What is often not appreciated is that influenza, in combination with its often complicating pneumonia, is the fifth or sixth most common cause of the death in the U.S. during a given year. The majorities of flu deaths occur generally among the elderly, especially over the age of 65, and result from complications most often involving bacterial pneumonia, but lethal infections may result from mixed viral and bacterial or pure viral pneumonias, as well. Deaths are counted against what is considered to be the average in a given winter, and based on these values, at least 10,000 deaths, occasionally up to 50,000, exceeded the average for each of 20 epidemics recorded in the U.S. from 1957 to 1987.(1) Without an epidemic flu outbreak, it is estimated by the U.S. Centers for Disease Control that the annual death toll for flu in the U.S. is about 36,000.(4) In a serious epidemic, however, deaths could likely rise to be 300,000 or more.
          Complicating the potential for serious deaths from flu is the fact that the disease is highly contagious. Influenza virus replicates in the respiratory tract. It is spread by aerosols of respiratory secretions created by coughing, sneezing, and speaking. Following its introduction, influenza begins with an incubation period from one to four days. Fever ensues rapidly and may be accompanied with a sore throat, dry cough, headache, myalgia, malaise, or anorexia. The disease may range from a mild pharyngitis to a complicating pneumonia leading frequently to death.
          Among the three immunologically distinct types (A,B, and C) of influenza, type A is the most highly pathogenic, producing dangerous complications, and subject to antigenic drift. This change in the immunogenicity of the virus makes existing vaccines obsolete and leads to continual requirements for new vaccines. Two proteins, hemagglutinin (HA) and neuraminadase (NA), associated with type A influenza are, in part, responsible for the pathogenic character of the virus. In birds, influenza, type A, is divided into 14 subtypes of HA (H1-H14) and 9 of NA (N1-N9). Various combinations of these sub-types are found in mammals, including humans. The potential combinations of the HA and NA sub-types provides for the very high level of antigenic drift. The next strain of virus, when it will strike, how widely it might spread, and how virulent it could be are all difficult to predict. Because it would be quite easy to intentionally manipulate the virus to produce a highly contagious and pathogenic strain of influenza, the potential to use influenza as a biological weapon must be considered. Although the public is unaccustomed to thinking of influenza as a serious disease, the potential for a deadly strain of influenza virus to develop – either naturally or through molecular manipulation – and spread rapidly and effectively is most possible.
          Although it is possible for zoonotic influenza (i.e., swine flu and avian flu) to be a concern, human-to-human transmission of influenza virus should be considered a real possibility. In May 1997, an individual died of H1N5 strain of Type A influenza. H1N5 had been previously known to exist only in birds.(5) Although the original transmission may have taken place from bird to human, human-to-human transmission was also taking place. Following a few deaths, it was realized that it would be necessary to slaughter hundreds of thousands of chickens in Hong Kong to limit a potential epidemic that could erupt to a pandemic. As predicted by professionals, this did not stop avian flu. During July of this year, China, Vietnam, and Thailand had avian flu outbreaks. Eight countries in Asia endured an avian flu epidemic killing at least 23 people during the early part of this year.(6)
          Is it possible for influenza to be used as an agent of bioterrorism? Human-to-human transmission of influenza offers the potential for the virus to be genetically manipulated to produce an agent that could produce a highly pathogenic, potentially deadly agent that could be spread using “human vectors”. Although the virus could be mostly incapacitating, without a vaccine for an unexpected strain of influenza virus, a devastating disease could be propagated in the U.S. or elsewhere. Since influenza could be initiated during any time, it is reasonable that a flu epidemic could be started during a season other than late fall or winter, when flu outbreaks occur almost exclusively. Thus, using human vectors with an atypical, highly pathogenic strain of influenza virus for which there is no existing vaccine to spread disease in the U.S. during spring or summer is a frightening scenario.
          Methods to limit the spread of the virus, ways to determine such an outbreak, and the logistical response to such a bioterrorism event need consideration. This is especially important in light of recent events where available vaccine for existing flu strains is limited and for strains of virus not likely to be as severe as the avian flu.

References

1. D.O. White and F.J. Fenner. “Medical Virology, 4th Ed.” Academic Press, San Diego, CA. 1999.
2. G. Kolata. “Flu” Touchstone Press. New York, N.Y. 1999.
3. W.J. Martone. “Influenza.” National Foundation for Infectious Diseases http://www.nfid.org/library/influenza/credits/index.html
4. J.M. Barry. “The Great Influenza” Viking. New York, N.Y. 2004
5. On-line Focus. “The Hong Kong Flu” Dec. 16, 1997 http://www.pbs.org/newshour/bb/health/july-dec97/flu_12-16.html
6. D. Normile and M. Enserink. “Avian influenza makes a comeback, reviving pandemic worries.” Science 305:321. 2004.