Image source: Getty Images.

Chances are that if you look out your window the leaves on the trees are beginning to change color and the weather is beginning to shift from stifling hot to something more manageable. Yes, fall is in the air -- but so is the unofficial start of flu season.

To be clear, flu season never actually ends, but neither the media nor the American public tend to discuss it much once the number of reported flu cases tapers off. Unofficially, flu season runs from October through May, with the highest number of cases recorded between December and February. This means that we're just a matter of weeks from an expected jump in the number of reported cases of the flu.

Yet there's quite a lot the American public probably doesn't know about flu season. Here are six facts about the 2016-2017 flu season that all Americans should acquaint themselves with.

1. Influenza/pneumonia is the eighth-leading cause of death in the U.S.

When the majority of Americans think about the flu, they envision a fever, a cough, a runny nose, and perhaps a few aches and pains. But for some Americans, the flu can be deadly. Children and the elderly often have weaker immune systems that leave them more exposed to the flu's more serious side effects. According to data from the Centers for Disease Control and Prevention (CDC), influenza and pneumonia combined were the eighth-leading cause of death in the United States in 2014.

Image source: Getty Images.

2. A flu shot cannot give you the flu

If there's one dominant flu myth that's persisted for as long as I can remember, it's that getting a flu shot is going to give you the flu, which simply isn't true. With the exception of one vaccine, which we'll get to in a moment, injectable flu vaccines use inactive strains of the influenza virus. These inactive strains still act as a teaching tool for the immune system and help reduce your chances of getting the flu, but you can't develop the flu from the shot.

The one and only vaccine that's the exception to the rule is AstraZeneca's (AZN -0.35%) FluMist, which is an intranasal spray that works well for needle-phobic people. FluMist does contain a live attenuated version of the influenza virus, but it's only enough to give you the symptoms of a cold in a worst-case scenario. You still, even with live but weakened flu virus strains, can't contract the flu from any of the available influenza vaccines.

3. Researchers are really just guessing (but doing a pretty good job of it)

When it comes to the nitty-gritty of deciding what types of flu viruses drugmakers should prepare for in the upcoming flu season, it may be a bit shocking to discover that researchers are doing nothing more than offering their best guess based on previously culled data. Despite just guessing, they generally have a pretty good track record of success.

Why can't researchers pinpoint the specific strains with 100% accuracy? Two reasons. First, drugmakers need months of preparation time to create the tens of millions of vaccines that the American public uses every year. By the time researchers know what strains are dominant in any given year, it would simply take too long for drugmakers to manufacture a vaccine to treat those strains.

Secondly, there are a large number of strain combinations to choose from, and trivalent or quadrivalent vaccines only target three or four strains. For example, influenza Type A has two proteins on the surface of the virus: hemagglutinin (HA) and neuraminidase (NA). There are 15 different subtypes of HA and nine different subtypes of NA. Type A influenza is typically what leads to the threat of a pandemic, but fortunately a number of influenza strains can be ruled out as they only affect other animals or birds and not humans. This still leaves researchers to do some guessing.

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4. It's still worth getting a flu shot

Even though researchers are merely offering their best assessment of what strains will be dominant in the upcoming season, they've been more right than not historically.

An initial report from the CDC issued in late February 2016 showed, based on its preliminary data, that overall influenza vaccine effectiveness (VE) was 59%. Keep in mind that VE refers to the reduced percentage of vaccine recipients that wound up heading to the doctor during flu season. VE has nothing to do with the success or failure of a vaccine in preventing recipients from developing influenza. Getting a flu shot could, in theory, prevent you from developing the flu -- but that's not the intent of the vaccine.

Researchers weren't nearly as lucky with their preliminary update in the 2014-2015 flu season. The late February update from the CDC showed a VE of just 18% for Type A influenza viruses and a VE of 45% for Type B influenza. Despite a lower VE, we're still talking about a sizable reduction in the number of vaccine recipients that otherwise may have had to go to their doctor or the hospital due to their flu symptoms. 

Long story short: getting the vaccine is usually worth it.

5. New quadrivalent vaccines offer a better chance of vaccine effectiveness

The good news for Americans is that a new series of vaccines known as quadrivalents began hitting the market in 2012. These could providing a higher VE than ever before. Unlike trivalent vaccines, which cover two Type A strains and one Type B strain, quadrivalents are designed for two Type A and Type B strains. Type B strains typically hit later in the season, which could provide for better overall VE on the back-end of the flu seaason.

Image source: Sanofi.

The dominant name in the flu vaccine industry is Sanofi (SNY 2.25%). The company's vaccine, known as FluZone, has previously topped the $1 billion mark in annual revenue, which says something given that IMS Health estimates industry sales at $1.61 billion and CSL suggests it's a $4 billion industry. As quadrivalent vaccines become more prevalent, Sanofi's quadrivalent FluZone could continue to grow its sales by a double-digit percentage.

What allows Sanofi's FluZone to sit well ahead of its rival therapies is its multiple dosing options. Whereas some flu vaccines are served up as a one-size-fits-all, Sanofi's FluZone is offered in a half-dozen dosing options. As an added bonus, it can be administered to children as young as six months, and one of its dosing options features a needle that's 90% shorter than a traditional needle.

6. FluMist is no longer a recommended flu vaccine

Finally, in something of a surprise, the 15-member CDC advisory committee in June recommended that AstraZeneca's FluMist not be used in the upcoming flu season.

Image source: U.S. Army Corps of Engineers via Flickr.

FluMist was introduced as a pain-free flu vaccine option 13 years ago, and in 2015 an estimated third of all flu vaccines given to children were in the form of the intranasal spray. However, upon deeper analysis by the CDC of the 2015-2016 influenza season, FluMist's quadrivalent vaccine was only shown to have a VE of 46% compared to the flu shot's VE of 65%. Even more damning, FluMist had a VE of 0% against one of the identified strains of flu to affect people last year. 

FluMist typically wasn't a large seller, with revenue totaling only $288 million in 2015 for AstraZeneca. This works out to about 1% of AstraZeneca's total sales. While AstraZeneca probably won't notice a sharp drop in FluMist sales in the upcoming season, children and pediatricians could feel the effect.