Health & Medicine

How the “Flu Blacklist” Explains Why the 2013-2014 Flu Season is Deadly

Tomás Aragón

In my professional role, I am interviewed by the media to explain why this flu season is so “deadly.” I have a more mathematical explanation here (which is challenging to simplify). However, Rob Roth from KTVU Channel 2 interviewed me and suggested the term “susceptible list” to describe the people who are still on the “list” to get infected with H1N1. For instructional purposes only, I am using the term “flu blacklist” to emphasize the consequences of our choice to get vaccinated or not.


When the novel influenza A (H1N1) virus was introduced into the human population in spring, 2009, all of us were on the H1N1 pandemic “flu blacklist.” Being on the flu blacklist meant we were marked for infection because of our susceptibility to this new influenza virus subtype.

How did (or do) we get off the H1N1 flu blacklist? We got (or get) immunized in one of two ways: natural infection or vaccination. When the vaccine became available I chose vaccination for me and my family. However, many people chose not to get vaccinated. Therefore, they chose to remain on the H1N1 blacklist until they get natural infection. Because H1N1 flu caused an illness not worse than seasonal (nonpandemic) flu, nonvaccinated people felt less pressure to get off the blacklist.

In spring, 2009, and the winter 2009-2010 flu season, the pandemic influenza A (H1N1) virus was the dominant subtype in circulation. However, this was not true for the 2010-2011, 2011-2012, and 2012-2013 flu seasons: influenza A (H3N2) was the dominant circulating subtype (see Figure 1).

CDC Influenza Positive Tests, National Summary, 2010-2013

FIGURE 1: CDC Influenza Positive Tests, National Summary, 2010-2013. Notice that H1N1 is decreasing and H3N2 is increasing. Eventually, this changes (see Figure 2).

This meant that people on the H1N1 flu blacklist were less likely to get off the list until H1N1 flu virus returned to be the dominant circulating subtype. H3N2 flu virus subtype has been in circulating since 1968 (46 years!); hence, the H3N2 flu blacklist is MUCH SMALLER than the H1N1 flu blacklist, since H1N1 has only been circulating since 2009!

CDC Influenza Positive Tests, National Summary, 2013-2014 (Week 5; Feb 1, 2014)

FIGURE 2: CDC Influenza Positive Tests, National Summary, 2013-2014 (Week 5; Feb 1, 2014)

Since 2009, H1N1 and H3N2 have been competing against each other, and which subtype dominates depends on available susceptibles. In Figure 1 we can see that proportion of H3N2 flu was increasing each year, while H1N1 was decreasing. This season H1N1 is back — with a vengeance! (see Fig. 2). Why? For two reasons:

1. The H1N1 flu blacklist very large, and

2. The H1N1 flu blacklist is large relative to H3N2 flu blacklist

Therefore, we are seeing many more flu cases (and deaths) compared to the last few seasons because the number of susceptibles is very large (H1N1 flu blacklist). This would be true even if H1N1 continues to cause mild illness as before, which is a reasonable assumption (see footnote).

Conclusion: From a population view, this H1N1 flu season is more deadly because we have many people on the H1N1 flu blacklist (i.e., susceptible), but not because the virus is more deadly.

Recommendation: Get off the flu blacklist: get vaccinated — the current flu vaccine covers influenza A (H1N1, H3N2) and influenza B. Because immunity wanes (even with natural infection), you should get vaccinated every year. In other words, we slip back onto the blacklist, albeit with some residual protection.


Footnote: In general, from an evolutionary perspective, it is not to the virus’ advantage to become more deadly: this is because very sick or dead people are not good transmitters. The only exception would be if a mutation (“antigenic drift”) simultaneously caused a dramatically increase in transmissibility (infectiousness) that could make up for the fewer persons circulating because of severe disease. The chance of this would be very small, but not impossible. Studies of the current H1N1 virus should clarify this.

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Comments to "How the “Flu Blacklist” Explains Why the 2013-2014 Flu Season is Deadly":
    • Pietra G Mueller

      Thanks – the article was informative (supported by clear graphics) and well-written. I just saw it now b/c I was doing a bit of searching for info on the 2013-14 influenza season.

      [Report abuse]

    • Retta

      My 33 yr. old daughter is in hospital 3 weeks now with flu. We were told she was probably going to die. She was at dr. office, admitted to hospital and put on respirator in less than 12 hr. She was pregnant and had c section to at least save the baby. Baby is doing well but it was 2 weeks before she was aware she had him. That let them use medicine they couldn’t before and I believe relieved some of the pressure on her lungs. Currently has a trach and has been moved to rehab center at hospital. 2 weeks she was on respirator. Get your shot. She was a runner and singer so had healthy lungs and eats healthy foods (Did before she got sick. Now has tube feedings til she learns to eat with trach). Doctors tried a lot of different treatments to find something that worked. Plus she is a red head and they apparently don’t respond well to a lot of sedation and anesthesia.

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    • Mersable

      Common sense and reading comprehension would indicate that I would get a shot to help my body to fight influenza. No absolute promise is ever given that it will succeed.

      I think that anti-biotics may be prescribed if a secondary infection sets in? Perhaps I am mistaken.

      I know three people who suffered pneumonia as secondary this season, and two with sinus infections, (one of those had double middle ear infection in adult). My first degree relative who had pneumonia as secondary had actually begun to feel better before she felt much worse.

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    • Hazel Cunningham

      Recent studies identify a genetic marker common to Asian populations to be associated with severe and/or life threatening outcomes from the 2009 “swine flu” novel A/H1N1.

      Whether the current ApH1N1 epidemic is disproportionately impacting individuals with this marker (IFITM3 CC gene variant (aka C/C Genotype)) is unknown. Whether immunization off-sets increased morbidity and mortality in individuals with this marker also is unknown.

      In any event, people of good will should press the research-medical establishment to expedite studies to answer these questions.

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    • The data in the figure I present are accurate (given limitations of disease surveillance data). My conclusions are tentative since new scientific studies will provide more insights. My main point is that the H1N1 pandemic strain was recently introduced in 2009, the number of susceptibles to H1N1 is large (compared to H3N2 which was introduced in 1968), H3N2 was the predominate strain for the past three flu seasons, and H1N1 is the predominate strain this season; therefore, these facts alone may be sufficient to explain the large number of deaths this season. Stay tuned for important research to clarify if H1N1 has evolved to be more virulent (“deadly”).

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    • Robert

      The article states that H1N1 is relatively new

      When the novel influenza A (H1N1) virus was introduced into the human population in spring, 2009, all of us were on the H1N1 pandemic “flu blacklist.”

      and

      …since H1N1 has only been circulating since 2009!

      However, wikipedia states that H1N1 is responsible for the 1918 flu pandemic.
      http://en.wikipedia.org/wiki/1918_flu_pandemic

      The 1918 flu pandemic (January 1918 – December 1920)[1] was an unusually deadly influenza pandemic, the first of the two pandemics involving H1N1 influenza virus (the second being the 2009 flu pandemic).

      [Report abuse]

    • You are correct that the 1918 pandemic was caused by another H1N1 subtype. Having the “H1N1″ designation means they have similarities in two surface proteins: hemagglutinin (H) and neuraminidase (N). The point is that when the new H1N1 was introduced into the human population in 2009, there was little immunity. Seniors seem to have some protection suggesting that exposure to old H1N1 early in the 20th century has conferred some protection.

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    • iskandar

      You are saying that if you do not get vaccine or illness you will die? I am sure it would have been triple what it is now. I know many in hospital and ICU, but common sense helps, too, and that is not pointed out.

      [Report abuse]

    • Charleen Kennedy

      I am not an epidemologist, but I am seeing a huge increase in mortality due to H1N1 in our rural area. I find that most do not get vaccinated. My concern is that the larger the number of people who do not get vaccinated increases the chances of mutations.

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    • The term “increase in mortality” can have two meanings: increase in the number of flu deaths in a population, or increase in the individual risk of dying given flu infection. The former can be elevated even if the latter is low and unchanged. This can happen when the susceptible population is large (H1N1 flu blacklist). I am asserting that susceptible population is large because H1N1 was only recently introduced (2009) into the human population.

      Yes, influenza A virus is constantly mutating, and any immunization (whether by natural infection or vaccination) will put selective pressures on the circulating virus strains. A mutation that enables infection and transmission among immune persons will give the new mutant strain a selective advantage.

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    • Tom Ogren

      Very interesting article. I’ve a question I can’t seem to get a clear answer for: I read that a flu shot is really only protective for about three months, and it was suggested that some people get a second one this year. Is this correct?

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    • Ayla Lawone

      I got the flu shot as required by my health care job, and got the flu ANYWAYS. It’s bad, I’ve been off a week , had 103 deg F x 3 days, it’s down to 99 now.

      I went to the doctor, got antibiotic (in my opinion worthless for a virus), a cough pill, an inhaler, and was told to stay on the OTC meds I was already taking: pseudofed, nite time cough relief. I have laryngitis, bronchitis, fever, malaise and extreme fatigue, I have a barking cough and chest is full.

      The author fails to tell people that the vaccination is a live virus. Getting the flu shot does not mean you will not get the flu, just a “less severe” case than if you did not take the shot at all. (Per our nursing director.)

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    • Sam

      The flu shot is a dead virus. Look it up.

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    • Vaccination reduces your risk of getting flu, but does not eliminate it.

      The injectable flu vaccine is inactivated virus (dead virus particles). However, the inhalable is a live weakened virus (http://www.cdc.gov/flu/about/qa/nasalspray.htm).

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      • Ed

        Ayla, Tomas’ article was written to address one question, and it was not the one that you brought up, sorry. All of your concerns about flu shots, their effectiveness and how soon they “work”, etc., are addressed on other areas on the Web (I just found them minutes ago). Writers can’t include everything in one article, but the Web can, and does.

        On the other hand, I’m very sorry that an MD gave you antibiotics, there’s little excuse for that….and also sorry that you got sick anyway. So did I. Tomas, I’m glad to have found this site today and I look forward to more of your writings. Best wishes to you both.

        Ed from SanRafael, CA

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    • Mary

      This is the strangest article I have seen so far. So basically you are saying that if you do not get vaccine or illness you will die?

      I know many who got shot and all have got very ill. It is not a good match at all. CDC is covering up the mutation. Plus most of the people who have died had underlying medical conditions. I almost went for vaccine, but I had flu at end of November, it was bad but I made it.

      Media has this thing so spun out it is ridiculous. Plus you cannot compare numbers this year as to last because California does not count Seniors. I am sure it would have been triple what it is now. I know many in hospital and ICU, but common sense helps too and that is not pointed out.

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    • Reader

      “So basically you are saying that if you do not get vaccine or illness you will die?”

      No, that’s not what the article is saying.

      “common sense helps too and that is not pointed out.”

      Your post is an excellent demonstration of the fact that reading comprehension is much more useful than “common sense.”

      [Report abuse]

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