Swine Flu Sends Mostly Under-25s to Hospital: CDC

Although why a minority of patients become so sick remains a mystery, new research indicates that H1N1 is different from typical seasonal flu viruses in crucial ways -- most notably in its ability to penetrate deep into the lungs and cause viral pneumonia.
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So far, the virus does not seem to sicken or kill people more often than the typical flu. But the pattern of people getting seriously ill is far different than in typical flu seasons. The elderly, who are usually most vulnerable, are generally spared; children, teenagers, pregnant women and young adults are the most common victims.

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While about two-thirds of U.S. patients who were hospitalized in the spring had other medical conditions, the CDC reported this week that an analysis of more than 1,400 hospitalized victims found perhaps half had no serious health problems.

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There appears to be no way to predict with certainty who may suffer serious, life-threatening complications, since some victims have had no other health problems.(Rob Stein, WaPo)

...virulence is not just a property of the virus. It is a function of the virus, the status of the particular host it infects, and the environment they are in. For whatever reasons (probably related to the immune status of different age groups), pandemic viruses either infect or infect in a way to make clinically ill a younger group than seasonal viruses. And so far this virus is able to do it at a time of year that seasonal viruses are not very active in the population, so the environment is different -- and changing as we get into the colder seasons where flu is traditionally more active. So putting the first and second snippets together, you have some of the reasons why we said in an earlier post that it was the epidemiology that was important, i.e., the pattern of disease in the population, here meaning the age distribution. If virulence depends on both host and virus, then shifting it to a different kind of host (younger people with no pre-existing experience with similar viruses) can modify virulence. And the change in seasons and social networks that come with it are a change in the environment.


Clearly things like pre-existing medical conditions is one, as is pregnancy and morbid obesity, are host factors. But now that flu season is coming on we are seeing more people without known pre-existing conditions getting seriously ill (third snippet). So the environment is changing. We've discussed quite a bit our lack of definite knowledge of the seasonal driver for flu (is it temperature? humidity? lack of sunlight and Vitamin D? start of the school year? a combination of some or all of these? something entirely different?...but whatever it is it changes throughout the year and that is a factor in virulence. So we are also seeing a concomitant change in the epidemiology (pattern) of serious cases in the population.