Mar 09 2010

H1N1 Update

It seems as if the wave of H1N1 pandemic flu has passed, so it is a good time to get up to date on the status of the pandemic. For background, the H1N1 is a strain of influenza A  that cropped up about a year ago. It was declared a pandemic by the World Health Organization (WHO) last Summer.

The pandemic spawned a number of controversies. The last H1N1 outbreak, called the “swine flu” (a bit of a misnomer) was in 1976. The vaccine for that strain caused Guillaine Barre Syndrome (GBS) in about 1 in 100,000 people vaccinated. Therefore with the roll out of the new H1N1 vaccine there were cries from the usual assortment of anti-vaccine and other cranks that the vaccine would cause GBS, even though the last 30 years of seasonal flu vaccine has not caused any such outbreaks (at worst the seasonal flu vaccine causes an extra one case of GBS per million doses, but even this is questionable).

There were also accusations that the flu pandemic was a scam created by Big Pharma to sell vaccines, and the real conspiracy nuts claimed that the vaccine was in fact designed to infect and kill people.

Meanwhile, there were questions (legitimate and nutty) about how severe the pandemic actually would be. Of course, no one could know until after it happened.So how bad was it? Here is the updated numbers from the CDC:

# CDC estimates that between 41 million and 84 million cases of 2009 H1N1 occurred between April 2009 and January 16, 2010. The mid-level in this range is about 57 million people infected with 2009 H1N1.

# CDC estimates that between about 183,000 and 378,000 H1N1-related hospitalizations occurred between April 2009 and January 16, 2010. The mid-level in this range is about 257,000 2009 H1N1-related hospitalizations.

# CDC estimates that between about 8,330 and 17,160 2009 H1N1-related deaths occurred between April 2009 and January 16, 2010. The mid-level in this range is about 11,690 2009 H1N1-related deaths

In the last 10 years the seasonal flu has killed on average 36,000 Americans. The numbers above are just for H1N1, and are on top of  seasonal flu numbers (although I have not seen any final numbers on the seasonal flu yet).

So the net effect of H1N1 was to give us an especially bad flu season, but not the worst-case pandemic that was feared. It should be noted that seasonal flu kills mostly those >65 years old, while the H1N1 killed disproportionately those under 65, and also was more fatal to pregnant women. Raw numbers do not reflect this difference.

How about the GBS fears? Cases of GBS were carefully tracked by the CDC and in other countries and there was no increase in GBS associated with the H1N1 vaccine.

While cases are dwindling, the H1N1 pandemic is not quite over. It may be burning itself out, but cases are on the rise still in Africa. This leaves open the possibility that it could come back around. We have already had two waves of H1N1, and a third wave is possible. Further, it is the later waves of such pandemics which may be the most deadly. There has been a new mutation identified in a Mexican patient – and that is the concern, that the virus will mutate to a more virulent or contagious form before it comes back around. It is also possible that this strain of flu will simply combine and synchronize with the seasonal flu.

Thankfully, the H1N1 pandemic was a bit of an anti-climax. While it did bring a particularly bad flu season, it was toward the mild end of the spectrum of predictions. But also, all the fear-mongering about the flu vaccine also fizzled. The vaccine was safe and effective and served to blunt the effects of H1N1.

We will continue to track H1N1 as it may have another act to play.


After I posted the blog the representative from the CDC I had left a message with got back to me, just to confirm my reading of the stats.

The figures posted above are the hospitalizations and deaths from H1N1 alone, but essentially there were negligible cases of seasonal flu this year. This is still a big mystery – we were expecting H1N1 + seasonal flu, but the seasonal flu never showed.

Speculations as to why: high vaccination rates and high rates of compliance with good hygiene and people staying home if they were sick. There is also speculation about the H1N1 “crowding out” the seasonal flu strains, but this is not an established phenomenon.

The bottom line is that the flu is unpredictable – chaos theory in action. It will probably take a year or two to sort out why things happened the way they did.

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