So here we are, closely approaching what is our typical influenza peak season in the northern hemisphere. However, this season lacks the substantive morbidity that we experienced last season, and that we typically observe, to a lesser extent, at this time each year.
I am posting some recent maps and graphs of influenza surveillance for this season.
This first map shows the distribution of influenza-like illness reported so far this season across the US. "Influenza-like illness" is a category of symptoms that helps us epidemiologists keep track of potential flu activity without having to identify the virus in each person with flu-like symptoms. This would be far too time consuming and costly to be useful for infection surveillance, and would prevent us from responding quickly to epidemics. Instead we use the clusters of symptoms such as cough, fever, headache, and malaise. It is important that we document how these clusters occur in time and space. So this is a critical component to the infection landscape of influenza.
Compare the map above to that below published by the Centers for Disease Control and Prevention. The CDC map below uses actual influenza cases for this season.
So we can see that first map, which uses flu-like symptoms to track influenza is similar to the second map, which uses influenza cases to track influenza. Therefore we can see this "syndromic" surveillance can be very useful for keeping watch on influenza even though it is not as specific.
We can also examine syndromic surveillance over time. The following graph shows how the clusters of flu-like illness have varied across each week during the last three years. Notice the seasonal spikes that correspond to influenza peaks each year.
The first peak shows that the 2007 annual influenza epidemic was at its height in January and February and was quite severe, while the second peak shows that the 2008 influenza epidemic was also in January and February, albeit with a slightly later start, but was much milder. Now notice the 2009 influenza annual epidemic. Not only was this peak much more severe with flu activity at four times the baseline, non-epidemic, flu activity, but it also began much earlier in the season (September) than is typical of the annual epidemics, which tend to peak in early winter.
There is a good reason for this 2009 anomaly. Last year we experienced a rare event with respect to how humans and the influenza virus interact. This event was a shift event. Shift events can lead to pandemics, which is exactly what happened last year. Let's talk about what this means.
Each year, the virus that causes flu changes its genetic coding a little bit due to random mutations. This annual minor change in the virus genome makes our immune system no longer able to adequately recognize the virus. In other words, the virus antigen, which allows the immune system to target the virus, changes slightly each year. This is the very same reason that we need a new flu vaccine each year. This year to year genetic process of change in the virus is known as genetic drift. It is a gradual, yearly process of random genetic mutation that results in maintaining ongoing susceptible persons in the population.
However, after many consecutive years of genetic drift, each consecutive season provides less and less susceptible persons in the population. Eventually, usually only once in several decades, new selective pressures begin to act resulting in bigger genetic changes within the virus. These major changes produce a new subtype of the virus. If both antigenic components of the virus undergo this change, almost everyone in the population will be susceptible to the change. This is why the result can lead to a global pandemic. This process is known as genetic shift. And in 2009, the world experienced a shift event, which established the new 2009 H1N1 virus, which is now a regular circulating subtype. Under genetic shift, the change in the viral genome is not due to random mutations alone. It also requires reassortment. Reassortment is a phenomenon in which different viral strains or subtypes that are capable of infecting different animal species, are present at the same time in a single species and experience a large exchange of genetic information between the strains or subtypes. The animal species in which this occurs acts as a "mixing vessel" for the different influenza viral genomes to come together and exchange and re-assort their genetic information. The influenza viruses that infect humans are not particularly stable in humans, and so humans do not serve well as the mixing vessels for the genetic re-assortment required for genetic shift. Pigs, on the other hand, provide an excellent reservoir for genetic reassortment. This is why the genetic shift changes that lead to global pandemics are generally derived from pigs and are often colloquially referred to as "swine flu".
So, what will this season bring in terms of the annual epidemics? Well, we are not at peak yet, so we don't know. And no one can really say with certainty before March what may or may not happen. I will keep you posted throughout this year's flu season.
Also, look for a posting in the coming weeks on avian influenza, or "bird flu".