We’ve been traveling again (and offline), so we’ll limit this to a few comments to put recent news into the context of things we talked about here recently (an excellent up-to-date status report can be found by DemFromCT at DailyKos). A good article by Rob Stein of the Washington Post highlighted some of the increasing anxiety of clinicians as flu season ramps up with pandemic swine flu in the northern hemisphere. Stein’s article is quite long, but I have snipped a few things from it:
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.
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.
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.
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)
You would be right to be somewhat confused by this, but this is no fault of Stein’s. The picture is confusing for everyone, including scientists and clinicians, who are giving real time impressions as events unfold in different places and different times. Flu is notoriously patchy, unpredictable and variable and some of what we are hearing is frustration in not being able to get a clear picture of what we are dealing with.
Consider two disparate pieces, the first snippet which says this virus is acting differently in the lungs than seasonal flu because animal experiments suggest it can grab onto cells deeper in the lung; and the one following, says that it isn’t making people sicker more often than seasonal flu but the ones that get sick are different. If the virus can get down deeper in the lungs, why isn’t it making people sick more often than the regular flu? We’ve discussed some of what is involved here often over the years (e.g. here, here and here for a few of many examples), but we’ll summarize some of it again.
One of the things we keep our eye on with any infectious disease is how virulent it appears to be. “Virulence” is a measure of the seriousness of the infection. The common cold is not as virulent as influenza, even though people with colds are both infected with a virus and in both cases there is a spectrum of severity. On average, you are much sicker from infection with influenza virus than infection with a cold virus. Bird flu is very virulent, with 60% of the recorded cases dying. Most flu has a much broader spectrum of virulence, with roughly (rule of thumb) a third of patients having little or no symptoms, a third feeling like they have some kind of bad cold or respiratory illness, and a third with full-blown, typical flu, some so serious they need hospitalization or have a fatal outcome. If you’ve even been in the last third, with classical influenza, you will never take flu lightly again. It’s miserable.
So what would make this virus more or less virulent than seasonal flu and is it? The most important idea here is that 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?; see some example posts here, here, here), 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.
The last snippet reinforces a constant theme here and with flu scientists everywhere. This is a very unpredictable disease and virus. As much as we know about it, and that’s quite a bit, we are very far from understanding its dynamics. We’re not even sure how much we don’t know.
Finally, last week I indicated my reasoning for getting both seasonal flu and swine flu vaccines. For starters, it would protect me against influenza B. But I also noted that no one could be sure that the seasonal H1N1 and H3N2 wouldn’t return, despite the fact all currently circulating viruses in the US were swine H1N1. What will or won’t happen when the usual peak of flu season occurs in January and February we don’t know. But for the doubters out there, here is something from the most recent WHO weekly update (#70):
Of note, nearly half of the influenza viruses detected in China are seasonal influenza A (H3N2) viruses, which appeared prior to and is co-circulating with pandemic H1N1 2009 virus. (WHO Update 70)
If swine H1N1 burns itself out here, there is still plenty of seasonal virus around in China. And that’s the traditional historical incubator for flu virus. Just thought I’d mention it. Again.