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influenza: too close for hindsight

Posted Jan 04 2010 5:00am
Many end-of-year medical commentaries for 2009 have naturally taken a look back at the 2009 H1N1 influenza virus, as I did, in part, last week. Unfortunately, some are losing sight of the fact that while we are currently in a lull in influenza activity, the "flu season" isn't over yet, and so drawing conclusions about the impact of the 2009 H1N1 virus is quite premature.

The US has currently seen two peaks of H1N1 activity, the first being shortly after its discovery, and the second ending just a few weeks ago. From the CDC's site, the current trends in positive influenza tests being reported tell the tale:

In addition to this, the CDC has reported a mid-range estimate of 47 million people infected with the novel H1N1 virus, and 9,820 H1N1-related deaths from April 14 through November 14. When compared to traditionally cited estimates that seasonal influenza typically infects 5-20% of the US population and is responsible for ~36,000 deaths annually, this seems to paint a picture that the 2009 virus is panning out to be overall less lethal than the typical seasonal varieties. It has even been suggested that perhaps we should stop vaccinating against the new virus, to allow it somehow to "overtake and replace" the traditional viruses in circulation for this reason.

These latter conclusions are just plain bad, let alone unscientific, for a number of reasons:

  • we're only halfway through the traditional influenza season - "Flu season" is tracked from October through May, so any numbers at this point are useful for tracking the disease progress, but too incomplete to make any conclusive statements from. Furthermore, seasonal influenza over the last couple of years hasn't peaked until February or March, so it's entirely possible that there's more activity in store for us ahead.
  • comparing pandemic influenza statistics to seasonal flu stats during a pandemic is akin to comparing apples to oranges - As the WHO rightly points out, the numbers for people who are infected by, or die from, season influenza are based on statistical models that look back on the affected population and influenza season as a whole. In contrast, during this new H1N1 pandemic, the primary data reported on are the number of positive tests, not estimates based on complete data. While the CDC has been reporting the number of people affected as estimates, their margin of error in the April to November numbers quoted above is huge due to the fact that the season is still unfolding (between 34 and 67 million cases, and 7 to 14,000 deaths). Accurate numbers to estimate from won't be available until we are well out of this flu season.
  • the novel H1N1 virus may return again, possibly in a more lethal form - Previous pandemics have taught us that there can be at least three "waves" of spread of a new influenza virus through a population, and that mutations the circulating viruses can pick up can make it cause more serious illness. While a post-holiday spike in cases that some predicted (due to increased travel, and increased clustering of people together) hasn't shown itself yet, this pandemic is spreading in a strikingly similar way to that of 1957, which had another peak in January.
  • it is impossible to predict how this will impact seasonal influenza viruses - The usual seasonal varieties of influenza have yet to show themselves, although as mentioned before, it is earlier in the season than they are usually seen. It is impossible to say at this point if or when they may surface, and eventually if the 2009 H1N1 will join or supplant them over the coming years, or simply fade away. The answer to this lies in numerous complex factors that we can't accurately model for right now, including viral genetics, population immunity from infections and vaccinations, and environmental components.

We may be in the eye of the 2009 H1N1 pandemic storm (or it may, in fact, have already passed us by), but it is still too soon to meaningfully analyze how "bad" it is on its own, or compare it to recent seasonal influenza. That said, this is an excellent time to look back at some of the human factors involved in the pandemic to date, and some lessons learned... which I will be doing shortly.
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