 |
   |
||||
|
|
|
 |
||
|
 |
 |
 |
||
|
in Publications: Font size:    
|
|
What Severe Fall 2009 Influenza Pandemic Wave?
As the U.S. Departments of Health and Human Services and of Homeland Security prepare at full speed for the potential arrival of a severe second wave of the influenza pandemic this autumn 2009, world pandemic influenza experts David Morens and Jeffrey Taubenberger are asking, “Severe fall 2009 influenza pandemic wave? What “severe fall 2009 influenza pandemic wave?” David M. Morens, M.D., and Jeffery K. Taubenberger, M.D., Ph.D., are senior members of the staff of the National Institute of Allergy and Infectious Diseases, National Institutes of Health, who question the assumption that pandemic influenza comes in waves (like seasonal influenza) in the August 12, 2009 issue of the Journal of the American Medical Association. (1) They have published a number of other important articles. (2-4)
Morens and Taubenberger challenge the traditional notion that a milder wave of illness (“herald wave”) precedes a severe wave of illness in influenza pandemics. First, they deal with the notion of a “wave” and then the notion of a “herald” wave. The notion of a “wave” entered into “common use after a highly-fatal global influenza pandemic that spread from Asia in 1889, say the authors. “The term originally was applied to as many as 4 annual and largely seasonal postpandemic influenza mortality peaks recognized in many large cities between 1890 and 1894,” they say. (3,5) A cursory look at a military graph depicting influenza and pneumonia mortality between 1911 and 1920 makes clear why observers called the mortality peaks “waves”--they look like waves, i.e., the periodic up-and-down flow of influenza-related deaths.
The concept of a specific kind of wave--a “herald wave”--originated with the 1918 pandemic, when some observers believed a premonitory, more benign, “herald wave” of what clinicians were calling “influenza” afflicted some people in spring 1918. Then followed in September 1918 a much more severe and lethal illness that clinicians were also calling “influenza.” The idea behind a mild wave preceding a severe wave of influenza illness is that “as new viruses begin to circulate in human populations they inevitably acquire mutations that increase transmissibility and virulence.” (5-7) In other words, the then-novel spring 1918 influenza virus acquired mutations during the summer that turned the fall 1918 influenza “wave” into a virulent nightmare. How pervasive was the idea that pandemic influenza started mild and then got serious? Dr. Richard E. Shope, the eminent virologist who first isolated the influenza virus in pigs (Rockefeller Institute for Medical Research) exemplifies the strength of this belief among scientists even in the 1950s. He said the following about the 1957-1958 influenza pandemic: “A second wave might be as disastrous as the epidemic in the fall of 1918, which killed millions of persons.” (8-9) The second wave of the 1957 influenza pandemic was not more disastrous than the first. Morens and Taubenberger note, “Many investigators working in and since 1918 have cited evidence for or against ‘spring waves’ and their protection against later pandemic waves.” For example, Dr. Shope said in 1957, “Adults who have apparently missed infection in the first wave of Asian flu will be particularly susceptible if a second wave develops…He added that those who had Asian flu during the first wave probably would be immune during the second one.” (8) The idea of mild waves and severe waves in pandemic influenza is captivating, but is also probably invalid, say Morens and Taubenberger. Why? The answer is because much of the evidence from 1918 is confusing and does not support the hypothesis. Defining influenza was confusing to physicians who cared for patients in the midst of the 1918 pandemic, as made explicit by Dr. Milton Hall in his chapter on respiratory diseases in The Medical Department of the United States Army in the World War: Volume IX, Communicable and Other Disease. (10-11) Eventually, most clinicians in 1917-1918 settled on the hypothesis that the spring outbreak of influenza was uncomplicated (i.e., several days of illness with complete recovery, fever, headache, aches, sore throat, scratchy cough, coryza) while the fall outbreak was complicated influenza—complicated, that is, by pneumonia, which killed around 3% of the patients with complicated influenza, according to the U.S. Army data. Of the 791,907 U.S. Army soldiers diagnosed with influenza between April 1, 1917 and December 31, 1919, 24,664 (3%) died. (11) Morens and Taubenberger say much evidence suggests a great deal of variation among the 1918 pandemic influenza experiences of different countries. “Different countries had anywhere from 0 to 3 waves or occurrences [and] the course and timing of [them] varied greatly. Most of the world had 2 occurrences, one around October-November 1918 and a second around February-March 1919,” note Morens and Taubenberger. (5) Furthermore, pandemic influenza recurrences sometimes showed increased severity (ability to produce fatality) and occasionally lessened severity over time, they aver. What does one make of this evidence, which conflicts with the idea of mild and then severe pandemic influenza waves? Morens and Taubenberger say, there is “little consistent evidence of wavelike behavior in the major influenza epidemics and pandemics of the past.” (12) Furthermore, “there is a general tendency of pandemics to quickly assume annual seasonality in temperate zones. The distinction between seasonal postpandemic recurrences and seasonal endemic recurrences seems to blur as herd immunity increases over time and viruses drift antigenically.” (12) What does this statement mean? The authors say that when a novel influenza virus makes its presence known on earth by whatever mechanism, the virus tries hard to infect every person, but fails because its level of transmissibility between humans is only intermediate. The virus tries again by recurring one or more times “before settling into the familiar pattern of annual or seasonal endemicity.” We do not know why this happens, they admit, but the “key to understating this phenomenon may partly lie in the strong modern-day tendency for influenza viruses, along with most other respiratory viruses, to favor cold-weather circulation in human populations, at least in temperate zones.” (13) Seasonal influenza occurs in waves, and a new pandemic strain simply joins the wavy pattern, so to speak. One can almost see the pandemic influenza virus of 1918 joining the ongoing march of seasonal waves of influenza in the graph above. What are the implications of this observation for today’s novel H1N1 influenza pandemic? Morens and Taubenberger conclude, “Considering the long and confusing track record of pandemic influenza, it is difficult to predict the future course of the present H1N1 pandemic.” Its recurrence in fall 2009 may be “only” a seasonal wave, not a “severe wave” characteristic of the debunked hypothesis that pandemics have herald waves, followed by severe waves, held by people like Dr. Richard Shope. Morens and Taubenberger have articulated an interesting possibility and now we wait to see what happens. Notes:
|
Copyright
2007 - SEMP INC., All Rights Reserved.