disease, human conditions, medicine

It’s That Time Again – the Flu Season’s Here

It’s long been hard for people to accept that they can be hurt, or even killed, by something they cannot see. But this is exactly what viruses do; they evade our body’s immune system, hijack our cells to create more viruses, and have consequently caused epidemics throughout human history. Influenza (also known as the flu) is the virus responsible for the worst epidemic in modern history; in 1918, half of the world came down with influenza. Within months, between 40 to 100 million people (or about 5% of the world’s population at the time), mostly healthy young adults, died from the “Spanish flu.” That epidemic is also why scientists and physicians were terrified in 2009; the influenza strain of 1918 was also H1N1. Luckily, the 2009 H1N1 influenza strain, the “swine flu,” didn’t prove to be as virulent as the 1918 strain.

H1N1 influenza flu virus
The 2009 H1N1 influenza virus strain. (Image credit: U.S. Government)

So why is the flu so effective, and why is there a different flu vaccine every year? The protein “coat” that surrounds the influenza virus is what our immune system uses to recognize and attack the virus, but the virus changes this coat all the time, making it hard for our immune system to identify. Part of why it changes all the time is because it doesn’t just live in humans; some influenza often benignly lives in the digestive tract of pigs, ducks, and other waterfowl. There it swaps genes with other influenza strains, and occasionally a very potent virus is created this way. Consequently, where people are often in contact with these animals new influenza strains can find their way into the human population, such as in many Asian countries.

So how does this season’s flu compare to previous ones, and – the question many people wonder – how effective is the flu vaccine against it?

To answer these questions requires an understanding of what the influenza virus is. There are three types of influenza (A, B, and C), but only two are dangerous for humans (A, which can infect other mammals and birds, and B, which only infects people). These types can be further sub-typed based on the proteins they have on their surface (specifically hemagglutinin and neuraminidase, abbreviated “H” and “N,” respectively). To fight off the virus, our immune system needs to recognize these surface proteins. It sounds simple enough, but influenza viruses are constantly changing, or evolving, these proteins, and this helps them evade the immune system. Different influenza virus strains are made by having different, modified (H and N) surface proteins.

Based on data collected by the Centers for Disease Control and Prevention, the most prevalent influenza strain this flu season has so far been the 2009 H1N1 (specifically A/California/7/2009-like, the infamous type A virus isolated in California in 2009), followed by a type A strain that’s H3N2 (specifically A/Texas/50/2012-like), and various B viruses. These strains closely match the ones that the 2013-2014 influenza vaccine for the Northern hemisphere should give protection against, specifically the same 2009 H1N1 and H3N2 strains, and some similar B virus strains. (Previously, the vaccine has been a trivalent one including protection against two A strains and a B strain, but this year, for the first time, a quadrivalent influenza vaccine is available – it includes protection against an additional B strain.)

It’s hard to predict how effective a vaccine will be, but generally when it’s a good match, a vaccinated person is about 50% to 60% less likely to get the flu (than if they weren’t vaccinated). The CDC reports visible impacts – last season, the vaccine was a good match (and about 44% to 56% effective) and it’s thought that the number of people who would have gotten the flu and been hospitalized was reduced by 14% due to vaccinations. This might sound like a small amount, but considering that even with vaccination efforts there were 381,000 Americans hospitalized last year, this means that more than 62,000 additional people would have been hospitalized if it weren’t for the vaccinations. It’s hard to tell yet, but the current season’s vaccine may be equally effective.

If you haven’t had the 2013-2014 seasonal flu vaccine yet, you may want to read an article from the CDC released just last week that encourages people in the U.S. to get vaccinated. Even if somebody’s young and healthy, it’s important to consider the health of others when deciding whether to get vaccinated – the most vulnerable individuals, the people who benefit most from the vaccination efforts, are children younger than 5 years old and people 65 and above.

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