The H3N2 Flu Virus: An Epidemic Of Our Time

Statistically speaking, it is likely that you or someone you know has come down with the flu this 2018 winter season. And it is not a coincidence. In fact, the influenza virus has caused an unusually large number of hospitalizations this year: we are officially at an all time high since the Center for Disease Control and Prevention (the CDC) starting recording the data in 2010. If you’ll recall, the United States had a similar scare back in 2009 with the swine flu epidemic, and we are now reaching similar numbers in terms of flu-like symptoms for the current season. In the third week of January alone, according to the CDC, over 4,000 people died due to the influenza virus or pneumonia. What’s most frightening is the current death toll of our pediatric population: 63 children have already died due to flu-related causes, a number which is sweeping the news, and continues to rise.

But why is this year different from those in the past? While the answer is multifaceted, I will do my best to break it down for you.

The influenza virus comes in four different types -- that is, type A, B, C, & D. Starting at the end of the line, influenza type D does not typically infect the human population, rather it is transmitted mainly through cattle. Influenza type C, although more likely to cause sickness in humans, usually presents as a fairly mild condition and heals quickly. Second to last is type B: while the influenza genus does infect humans more than the other types, it is as yet relatively tame in its disease process, and due to its slow mutation rate, humans are typically inoculated and protected from an early age.

Type A is that which is of primary concern for us humans. Although wild birds were the original host of type A influenza virus, it has, in the not so distant past, begun to infect humans. To find out why type A viruses are so concerning, let's look a bit deeper.

Each of the influenza strains can be further divided into subtypes, called serotypes, which are given names based on two different types of proteins which are present on its surface: hemagglutinin (represented by the capital letter H) and neuraminidase (represented by the capital letter N). There are 18 different hemagglutinin subtypes (and therefore, subtypes H1-H18 exist), and 11 different neuraminidase subtypes (designated by N1-N11). I am sure by now you can see where this is going!

Historically, outbreaks of the type A influenza virus of different subtypes infected various populations. A couple of examples are the H5N1 subtype, which was the culprit behind the bird flu of 2004, while H2N2 caused Asian flu back in 1957.

Here are some you may be more inclined to recall: the H1N1 subtype was behind the Spanish flu in 1918, and, as I am sure you will recall, the Swine flu epidemic in 2009. Finally, a subtype with which we are about to become more familiar is the H3N2 influenza virus which caused the Hong Kong flu in 1968. And it is the H3N2 virus that is now behind the 2018 influenza epidemic.

Why is this strain so lethal when compared to the other types? It comes down to a basic reality of immunological science: the proteins on a virus’s surface can mutate very slightly, but just enough to throw the immune system off. It is the mutation rate of a virus subtype, then, that dictates its evasiveness to human vaccination. Influenza virus B (mentioned earlier) has a slow mutation rate which indicates its relative tameness. Influenza A, however, and especially the H3N2 subtype, mutates at a much higher rate.

So too, as I am sure you are aware, a yearly influenza vaccine is released to the public in an attempt to squelch the worst of that season's most prevalent virus strain. However, the speed at which the H3N2 subtype mutates creates a serious issue for the scientists making the seasonal vaccine: the virus could mutate while a vaccine to the original strain is being distributed. On a practical level, such a mistimed mutation could result in an entire population that is vaccinated with the wrong strain. This can lead to, you guessed it, an inflated influenza season. While the precise reason for the 2018 H3N2 influenza strain becoming an issue is a bit more involved, the vaccine this year was, in essence, far less effective than previous years’ vaccines have been.

And it gets even worse. Not only does the mutation rate of the H3N2 virus increase the likelihood of an outbreak, but the strain is intrinsically more deadly to specific populations than its cousins due to a theory called “original antigenic sin” or “imprinting”. The concept is essentially based off the idea that a person's immune system is more geared to viruses that they’ve seen in the past. The H3N2 virus first appeared in 1968 as I noted previously. The elderly (or those currently over the age of around 50) did not see this particular strain of the virus growing up and are therefore (theoretically) at a greater risk than the general population. The less effective flu vaccine this season does not bode well for the geriatric population (as well as the pediatric population, as previously noted).

To be sure, the flu vaccine should still be taken. While the vaccine is less effective than years in the past, that does not indicate zero efficacy: the shot still decreases the chances of coming down with the flu; remember, there are other strains lurking around as well. Moreover, through a concept called “herd immunity”, receiving the flu vaccine helps protect those around you who may not be able to get one, such as young infants, the elderly, and those who are otherwise immunocompromised. Yet, we are often presented with a classic case of the bystander effect: if individuals believe that everyone else will get the flu shot, then those individuals may be less inclined to receive it themselves. Please, if you are medically able, get the vaccine -- it will benefit not only yourself, but those around you as well.

I sit here writing from a NYC subway station, in consideration of our predicament; alarms and announcements from the CDC are echoing throughout the caverns, informing the citizens as to proper hygiene, and the requirement to promptly see a physician should one be experiencing flu-related symptoms. The movie-like scene, however frightening it may be, should only emphasize the seriousness of this years flu season: get the flu shot if you can (it is not too late!), wash your hands often (and keep your hands off your face), eat healthy, get plenty of exercise, and sleep well; we are still neck deep in the season, but it should be on the mend shortly.