As someone who briefly dealt with the flu this season, I can gladly say "good riddance!"

While the flu isn't gone by any means -- some 4,600 confirmed cases were reported during week 3, from Jan. 18- Jan 24, according to the Centers for Disease Control and Prevention -- influenza-positive tests have plummeted over the past four weeks from a peak of more than 12,800 cases.

Source: Centers for Disease Control and Prevention.

In other words, it would appear that the worst of flu season is likely over a bit earlier than normal, and I doubt people are going to be sorry to see it go.

However, it should be noted that just because the number of confirmed flu cases has fallen, it doesn't mean we're out of the epidemic zone yet. During the third week of 2015, the CDC reported that 9.1% of all deaths according to the 122-City Mortality Reporting System were due to pneumonia and influenza, well above the 7.1% historic threshold for week three. Thankfully, pediatric deaths have totaled just 61 in the 2014-2015 flu season, compared to 109 in 2013-2014 and 171 in 2012-2013. 

Source: Centers for Disease Control and Prevention.

This year's flu vaccine was a dud
This year's flu season was a particular disappointment because of its low vaccine effectiveness of just 23% (at least based on the CDC's early flu season estimates). To be clear, the flu vaccine isn't a guarantee that's designed to prevent people from getting the flu. Instead, it's designed to train your immune system to defend against certain strains of the flu virus, potentially protecting you from getting the flu and reducing the severity and duration of your flu. It's aimed at keeping generally healthy adults and a small percentage of the elderly out of the hospital so doctors can focus their care on those with weaker immune systems, such as the elderly and children, who really need medical care.

This year's flu season threw a curveball at researchers and vaccine manufacturers because the H3N2 strain emerged well after researchers had told drug developers what strains to target. This left consumers largely unprotected from the H3N2 subtype, but still in good shape to weather the latter stages of flu season, when B-type influenzas tend to pop up.

Source: Flickr user William Brawley.

Flu vaccines are an inexact science
"How can this happen?" you ask? It has to do with the fact that picking out which strains will predominate in the upcoming year is nothing more than educated guesswork. There are way more than four different influenza subtypes, but a quadrivalent vaccine will only protect an individual against two influenza A type strains and two influenza B strains. This means researchers have to use whatever clues they have to inform vaccine makers what strains to target.

We also have to remember that drug developers need to begin making vaccines months in advance of the flu season to ensure that there's enough supply to meet consumer demand. This means, as we saw this year, that drug developers might be producing a vaccine that may not be targeted against a predominant influenza subtype when flu season hits.

Lastly, vaccine makers have to be cautious about how much of a vaccine they produce. Viruses are prone to mutate, and they can come and not return for years at a time, leaving some vaccine producers sitting on an unneeded and unprofitable inventory of influenza vaccines.

This vaccine maker may thrive
Even though this year's vaccine provided an unexpectedly low level of protection, it still kept 23% of people from either developing the flu or experiencing symptoms severe enough to require a doctor's visit.

However, what this year's early start to the flu season may have done, especially since the CDC referred to deaths associated with pneumonia and influenza as an epidemic, is encourage people who wouldn't normally get a flu shot to get one.

Source: Sanofi.

Although it's by no means a concrete measure of causality, I'd point to Merck's recent results, wherein its MMR-II measles vaccine saw sales soar by 34% in the fourth quarter over the previous year. I suspect this increase is no coincidence given the rash of recent measles outbreaks in the United States. By a similar token, flu vaccine sales skyrocketed with the swine flu scare in 2009. I suspect this all points to one thing: Sanofi's (NASDAQ:SNY) FluZone having a strong year.

To be fair, it's not as if Sanofi's FluZone ever really has an off year. The vaccine typically outsells all of its competitors combined! But there's a reason for its regular outperformance. For starters, it's offered in a half-dozen dosing options, allowing it to be administered to children as young as six months old. People aren't one-size-fits-all, and FluZone's many dosing options take care of that. FluZone can also be administered with the use of a needle that's 90% shorter than a traditional needle. For needle-phobic people like myself this is a nice convenience.

Just this past week Sanofi reported a 16.2% increase in total vaccine sales, which it suggested was tied to strong performance of U.S. influenza vaccines and emerging market growth. Further, Sanofi notes that FluZone High-Dose for the elderly and FluZone ID, its microinjection system which uses a transdermal delivery, accounted for 88% of all U.S. sales, up from 41% in 2013.

I'd suggest it's quite possible that FluZone could wind up exceeding its 2013 sales total of nearly $1.1 billion, which could add a small but welcomed boost to Sanofi's top- and bottom-line figures in 2015.