An Odd Flu Season, 2009

The Swine Flu – H1N1 – caused quite a panic around the globe. This flu – though recognized to be among the milder of seasonal influenzas – had one terrifying trait: it was said to be more deadly for otherwise healthy children, or perhaps older children (though not in California).

Reports of childhood fatalities were the focus of much media coverage of H1N1, with an emphasis on flu deaths among healthy kids, although even contemporary reviews noted that most of the kids who died had significant underlying medical conditions that made them more vulnerable. Subsequent reviews also noted the presence of major complications like MRSA. Viewed in isolation, the increase in childhood mortality with H1N1 was remarkable – seasonal influenza rarely kills those under the age of 19 (they are about 1% of annual flu deaths), and with H1N1 about 14% of deaths were among those under the age of 18.

But it wasn’t the only remarkable thing going on. A rigorous H1N1 sureveillance program implemented by public health authorities in the Australian state of New South Wales found some interestings stuff by tracking patients who entered the hospital with “flu-like” symptoms and were tested for H1N1.

Not surprisingly, given the grave concern about the virus, emergency room visits for flu-like symptoms went way up during flu season. Turns out that even at the height of the flu season, no more than 60% of patients entering a hospital for flu-like symptoms had the flu at all, and most of those had Influenza A, not H1N1. At no point during the summer did more than 22% those in the hospital with flu-like symptoms actually have H1N1.

There is another oddity in the New South Wales data. They tracked confirmed H1N1 patients that then wound up in intensive care – meaning got really sick. From the news coverage we would expect that there would have been more older children in this category. But there were practically none – children 10-14 were virtually unrepresented in this group (a rounded average of 0/100,000 population). Children 15-19 were only slightly more likely to get sick with H1N1 (1/100,000).  The group who fared worst were patients 55-59 (4/100,000), followed by those 50-54, 40-44, and 65-69 (all 3/100,000).

Is there data in the U.S. also showing H1N1 disproportionately hit the middle-aged? Sure, there is California data showing that H1N1 was significantly more deadly for those 50-64 than any other age group. Their death rate was twice that of H1N1 patients over the age of 65. Remember, in a normal flu season, around 90% of seasonal flu fatalities are in that 65+ age group.

There is also this data from the CDC showing that nationwide the age group with the most deaths was actually those 25-49.

How is it possible that adults, especially those in middle age, were fairing so poorly with H1N1, but news reports focused on the danger to children? Maybe the high prevalence of co-morbidities in the adult victims of H1N1 factored into the reaction – they tended to be very overweight, diabetic, with heart and lung problems. Or maybe there was just something so terrifying (and thus compelling) about a story of a healthy child perishing from the flu – even if this story missed important parts of the larger trend.

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