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Michael: Do you feel that chill in the air? In a lot of places around the world, including North America and Europe, it’s that lovely time of the year known as “flu season.” And each year, we try to fight flu season with the influenza vaccine. Every season is kind of a whole new battle for researchers, because flu viruses evolve quickly. This year is no different.

The fact that there’s a flu season means doctors and healthcare organizations can prepare for it. But why is there a flu season at all? Why isn’t there just flu flying around everywhere all year? Well, there are actually two flu seasons every year, because the Northern hemisphere and the Southern hemisphere experience winters at opposite times of year. So here in the United States, for example, flu season lasts from about November through March.

And countries in the Southern hemisphere typically face the flu between May and September. The flu virus does circulate year round, but the number of cases rapidly increases during these peak months, giving influenza its own holiday season to celebrate. Research suggests that winter months are ideal for influenza transmission because they bring a combination of cool air and low humidity.

Influenza virus is transmitted through droplets from a sneeze or cough that are carried in the air. When temperature and humidity are low, these droplets can fly easily through the air without much resistance. But in warmer and more humid climates, the extra moisture in the air makes the droplets bigger and heavier, and therefore more likely to fall out of suspension in the air.

So the virus can’t travel very far. And of course, cooler temperatures also mean people tend to stay indoors, increasing the number of potential bodies in an enclosed space for the virus to infect. Makes sense, right? But even if the influenza keeps going from one hemisphere to the next, why do we need a different vaccine every year? Each flu season is unique, because the influenza virus mutates from year to year.

There are three major types of influenza: labeled A, B, and C. And influenza A viruses are further categorized into subtypes, depending on the two proteins that coat their outer shells. There are the HA, or hemagglutinin [hee-muh-glue-tuh-nin] proteins, and the NA, or neuraminidase [new-rah-min-uh-dace] proteins.

These proteins act like beacons to a host cell, tricking the cell into letting the virus attach and infect it. There are different subtypes of these proteins, H1 through H18 for the hemagglutinins and N1 through N11 for neuraminidase. So, the different combinations are why we talk about flu strains in terms of, say, H1N1 or H3N2.

But these HA and NA proteins mutate very easily, through a process called antigenic drift. These small changes don’t alter the virus very much, but after some time, the changes accumulate, and can result in a new strain of virus that acts in completely different ways than the original did. And sometimes, in rarer cases, genetic mutations in the virus can cause big, dangerous changes.

This is called antigenic shift, and it can result in a brand new combination of genes that make the virus much more infectious. You might remember the antigenic shift that happened back in 2009. That’s what created swine flu.

So, with different viruses mutating all the time, how do we know which strains are the one we should get vaccinated against this winter? Four strains are selected based on data collected throughout the year by health organizations monitoring flu activity in various countries across the globe. They report it to a network run by the World Health Organization, which then compiles the data to predict which strains will be likely be circulating during peak seasons and come up with recommendations for that year’s vaccines. Since the Northern Hemisphere and Southern Hemisphere tag-team winters, usually the data collected during one flu season will help inform the other hemisphere about which strains are doing the most damage, and if the vaccines were effective or not.

Flu vaccines are actually developed months before peak season starts, to allow time for them to be made and tested. But that means that sometimes the viruses that are circulating change while vaccines are being produced, so the vaccines might end up not being as effective. So, what strains are we up against this year?

There’s A/California/7/2009 (H1N1) pdm09-like virus, whose really long name comes from things like its type, where and when it was first isolated, and the subtype of its HA and NA proteins. It’s an influenza A virus isolated from California that’s closely related to the strain that caused the 2009 H1N1 pandemic.

Then there’s A/Hong Kong/4801/2014 (H3N2)-like virus... And a strain called B/Brisbane/60/2008-like virus which comes from a line of B viruses from Victoria, Australia. It gets a slightly shorter name because influenza B viruses aren’t categorized by subtype. In the quadrivalent vaccines like the nasal spray version, a fourth strain is included and is called B/Phuket/3073/2013-like virus descended from the Yamagata, Japan lineage of B viruses.

So just like spring, summer, fall, and winter, flu season is an inevitable season that comes back year after year. But scientists do their best to fight it by working year round to keep us up-to-date and protected from the ever-changing flu. So if it’s almost winter where you live, go get your flu shot if you haven’t yet, and brace yourself, flu season is coming.

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