Effect Measure

Swine flu news items

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.

[snip]

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.

[snip]

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.

[snip]

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.

Comments

  1. #1 Anna K.
    October 18, 2009

    I read that article as well — I also seem to recall that he said women in general are now considered to be more at risk, not just pregnant women, which surprised me.

    Thanks for your continued coverage on swine flu.

  2. #2 Elisabeth
    October 18, 2009

    Since the immune response plays an important role in the severity of illness with novel flu, perhaps the fact that women’s immune systems are stronger than men’s has something to do with it. And pregnant women. Thoughts?

    http://www.sciencedaily.com/releases/2009/05/090511180740.htm
    ScienceDaily (May 12, 2009) — When it comes to immunity, men may not have been dealt an equal hand. The latest study by Dr. Maya Saleh, of the Research Institute of the McGill University Health Centre and McGill University, shows that women have a more powerful immune system than men.

  3. #3 revere
    October 18, 2009

    Elisabeth: I rarely speculate about things like that. I prefer data. The question whether being female is a risk factor is still unclear. Even if true, there are too many possibilities.

  4. #4 Paula
    October 18, 2009

    I had noticed the Stein piece too. You might send him (and his editors) a copy of your page recognizing its excellence. Many recent news items are also commenting on delayed (swine flu) vaccine distribution delays, citing quite mixed causes–any thoughts on this? I know people are strongly reacting. Enough to use the adjuvants here, finally?

  5. #5 revere
    October 18, 2009

    Paula: The reason I almost always give by-lines in my links is to recognize good journalism. The papers are tracking the blog already. Current public responses about whether they will get vaccinated or not will depend quite a bit on their experience. If there is a lot of flu where they are they will be more likely to get it. We’ll only know later what actual behavior is. But Dem’s piece (first link in the post) has a good review of this.

  6. #6 Sungold
    October 18, 2009

    Revere: Love your blog! I rarely comment but always read.

    Regarding the start of the school year as a factor: I teach in higher ed (at a large-ish state school in Ohio), and we seem to be seeing more flu than is usual for this time of year. I realize this is anecdata, and it might well be warped by the new university policy of encouraging students to skip class when they’re sick. Even so, I’d be very curious to know if the pattern we’re seeing here is repeated in other universities.

    If so, perhaps this would suggest that the start of the school year is less important for seasonal flu – which ramps up more slowly than the swine flu has done – and more important (in relative terms) for novel flu viruses, where young people’s immune systems are naive.

    Just a half-baked thought, but maybe grist for future investigations?

  7. #7 don s
    October 19, 2009

    Is it perhaps fair to characterize it like this:

    This is a moderately virulent influenza (less than H5N1, more than seasonal) that is mostly not infecting the highest mortality risk population of the elderly. (Presumably because of some natural influenza infection or infections they had over 50 years ago. If they caught it it would likely be of high virulence to them.)

    Also, I still wonder, any epidemiologic speculation why seasonal influenza usually has a dip in percent of outpatient visits in January before its big surge?

  8. #8 revere
    October 19, 2009

    Sungold: Yes, universities are reporting a lot of flu, although it depends on what part of the country you are in. My impression is that the southeast and northwest are particularly hard hit and the northeast less so, but that’s just an impression. Regarding whether certain factors are more important for this flu than seasonal flu, the answer is a “general”, yes. That’s because what we see on the surface is the product of complex interactions below the surface between various factors in the hosts, the virus and the environment.

    don: Regarding the January issue, it differs by year and is subject to reporting vagaries so I am not confident it exists, but it may. We just don’t know at that level.

  9. #9 Paula
    October 19, 2009

    Don S., you say “mostly not infecting the . . .elderly. . . .If they caught it it would likely be of high virulence to them.” I have seen figures indicating 4x higher risk of complications, with the new flu, for persons over 65; is this the figure you mean? Or other data (which)? Thanks.

  10. #10 betty
    October 19, 2009

    College/University data can be found here: http://www.acha.org/ILI_Cumulative.cfm

  11. #11 Don S
    October 19, 2009

    Paula,

    I did not have specific data when I posted and recognize the difficulty of collecting it. OTOH CIDRAP puts it like this – http://www.cidrap.umn.edu/cidrap/content/influenza/swineflu/biofacts/swinefluoverview.html – “Persons aged 65 or older (the risk of infection in this group appears to be lower than for other age-groups, but once people of this age become ill they are at increased risk of complications)”

    UpToDate http://www.uptodate.com/home/content/topic.do?topicKey=pulm_inf/18836 gives these odd numbers:

    “rates of reported cases per 100,000 population … ≥65 years — 1.3 … Hospitalization rates per 100,000 population … ≥65 years — 1.7″

    Hard to understand how there were more hospitalized with it than who had the disease …

  12. #12 Paula
    October 19, 2009

    Perhaps different reporters, at figures’ origins? I agree, very strange to have higher percent hospitalized than cases for this (65 and up) group. Haven’t looked so carefully at the others for puzzles/glitches; have you, then?

  13. #13 Dylan
    October 19, 2009

    Don, Paula: Likely confused reporting.

    And there is stuff coming out of the CDC, now, that suggests that they are doing things to avoid recognizing the convergence of the “Influenza Pneumonia Syndrome” numbers with the confirmed H1N1 hospitalization and confirmed mortality figures. The confirmed H1N1 hospitalization and confirmed mortality figures will subsume the “Influenza Pneumonia Syndrome” figures very shortly. That’s a given. The last weekly report from the CDC confirmed “no” seasonal influenza infections in the course of a couple of thousand confirmed cases of “A” infection, out of roughly 13,000 tests. There were 15 confirmed cases of “B” infection, if I remember correctly. That tells us something.

    We’re looking at a very bad fall, and perhaps a worse winter, here.

  14. #14 Hank Roberts
    October 25, 2009

    Can you say more about pneumonia vaccine and age?

    I’ve been put off for several years, told several years in a row that it’s only appropriate for 65-and-over because it quits working after a decade or two and can’t be repeated, so they don’t want to do it too early.

    I’m not sure if “they” who doesn’t want to give it is Cigna or the doctor, and — high turnover in primary care doctors– I haven’t seen the same MD two years in a row for a while.

    Now I see I can just go to Walgreen’s and get it for the asking at age 60, never mind what the doctor/insurance says.

    Hmmm.

    (I know you can’t give advice and am not asking it specifically, just generally what’s the current best idea given the current flu? Or, what would/did you all do?)
    I gather from comments it may be recommended even for teens — if so cites or advice pointers please so I can pass those on to inlaws with teenagers.

  15. #15 revere
    October 25, 2009

    Hank: I got it (although I’m of the age where it’s always been recommended) but I would advise it for anyone. You only need to get the adult version once. This is general advice. If you have any risk factors, then even CDC advises it.

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