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).
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!
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.