Tag Archives: WHO

Daily Flu Counts

The bad news is that cases of novel 2009 influenza A(H1N1) continue to increase. Data from WHO Epidemic and Pandemic Alert and Response (EPR), Influenza A(H1N1) - update 43 — 23 May 2009:

The good news is that the spread appears to be sub-exponential at this point.  Exponential growth will appear linear on semi-logarithmic axes.  Here I plot the natural logarithms of these same case-count data against the date. We can see a distinct (negative) concavity, indicating that the growth in confirmed cases is sub-exponential.  The usual caveats about under-reporting and the lag between infection and reporting dates apply, but this is a modicum of good news.

The austral flu season will be heating up (as it were) soon enough. Once again, it seems only prudent to me that the richer nations of the north help poorer nations, who are about to get hit, with efforts to contain the spread of novel A(H1N1).  Given the relative genetic homogeneity of this novel strain, choice of a strain to include in a vaccine is straightforward (if a little late for the beginning of the northern flu season).  If we can minimize the intensity of antigenic drift (despite the name which might imply random change, this is directional selection away the ancestral antigenic type in the presence of multiple circulating strains) by minimizing the number of cases in the south during their flu season, perhaps we can dodge the bullet of an extremely high-mortality pandemic.

More On Flu

There is a nice video piece at the New York Times website done by science reporter Donald G. McNeil Jr.  In it, he makes a number of important points that I have been trying to emphasize in my latest posts on the topic. McNeil is to be congratulated.  This is the kind of reporting we need now and in the coming months on swine flu.

The New Scientist also has an editorial (which I just found because I'm behind on my RSS reader) which notes the distinct possibility that H1N1 could come back with a vengeance this Fall.  Note that most of the deaths in the pandemic of 1918 occurred in September of 1918 even though the first cases were reported in March of 1918.  The pandemic of 1918 (which was really the pandemic of 1918-1920) killed 50 million people, perhaps as many as 100 million.  The world population in 1920 was 1.86 billion, which means there were around 1.78 billion or so in 1918.  The case fatality ratio for the 1918 flu was >2.5%, and perhaps as high as 5%, which means that 25-50 people died out of 1000 infected with the flu.  All in all, this means that anywhere from 0.5% to 2% of the world's population died during the pandemic of 1918 (though if 100,000,000 people really died, then the overall world mortality rate was actually over 5%!).  The following figure (from Taubenberger and Morens 2006) shows the time series of deaths from flu for 1918-1919.  

Taubenberger and Morens (2006), Figure 1.

The most striking feature of this figure is the pronounced spike in mortality in the Fall of 1918.  We are currently a month before this time series starts in our current potential pandemic. Note that the death rate in June of 1918 was not too dissimilar from the mortality rate estimated from Mexican outbreak data by Fraser et al. (2009).

The New Scientist also reports that a flu vaccine incorporating the new A(H1N1) is unlikely to be available by the Fall of next year.  This is, of course, distressing news.  So, what can we do?

It seems to me that the best plan is to follow D.G. Margaret Chan's call for international solidarity.  She has rightly noted that the people likely to be hardest hit by the an H1N1 pandemic (or any other infectious disease for that matter) are the citizens of the world's poor countries.  They are the ones who bear 85% of the infectious disease burden, after all.  So, why should we in the developed north care about this other half (homage to Yogi Berra intended)?

For the time-being the strain of A(H1N1) is relatively benign (just as it was in 1918 at this point), but influenza has an incredible capacity to mutate, recombine with other co-circulating flu strains, and respond to selection (the part that you don't typically hear about in news reports). Let us not forget that there is currently another highly pathogenic strain of flu out there. Highly pathogenic Influenza A(H5N1) -- remember bird flu? -- has an overall mortality rate of approximately 60%. Yes, that's an order of magnitude greater than the high-ball estimate for the 1918 flu.  Of course, this variant of influenza has only infected a total of 424 people in the world since 2003.  141 of those have been in Indonesia (where 115 have died for a case fataility ratio of 81.5%).  We have gotten very lucky so far with H5N1 because it is not efficiently transmitted from person-to-person.  The emergent H1N1, however, is.  It's basic reproduction ratio is substantially greater than that of seasonal flu, which indicates it is very efficiently transmitted. God help us if a recombinant strain with the pathogenicity of bird flu and the transmissibility of swine flu were to evolve.

This suggests to me that a little bit of financial and technical assistance from the north to the countries of the south might be a very good investment at this point. Let's help developing countries entering their flu season control swine flu to the absolute best degree we can manage. Minimizing the number of cases also minimizes the evolutionary potential of the virus -- fewer infections, fewer opportunities for mutation and subsequent selection. I realize that we are in the throes of a major economic crisis, the likes of which we haven't seen since 1929.  But, do you have any idea what losing 5% of the world's population would do to the economy?