Wow ... there are a lot of misconceptions about what the flu shot actually is. I would be happy to clarify this for a lot of people on here. Whether you choose to have it or not is your decision, I'll leave that part up to you, though.
So the flu shot itself is actually based on the opposite hemisphere's flu season. So if you are in the northern hemisphere (as most people on here are), your vaccine is based on what we experience in the southern hemisphere during our flu season (and vice versa). The reason for this is that each hemisphere's flu season (conveniently enough) is at exactly the opposite times of the year. This, however, only provides a guide as to what they might expect to see. It isn't perfect and there are plenty of reasons why this is that I'll explain a little later on.
Your standard flu vaccine is what is known as trivalent (tri meaning three). There are some others that are quadvalent, as well as a few different trivalent subtypes of vaccines. And there is also a lot changing as I know that in Australia here we are phasing out the trivalent and opting for the quadrivalent one instead (mostly because these have better coverage as well as are the recommendation for infants as well as elderly people). But I'll just focus on the most commonly used one. So the trivalent one covers three of the most common subtypes of influenza, two types of influenza A and one type of influenza B (with the quadrivalent covering two types of influenza B, instead of just one). These are selected as I mentioned based on what is seen during the influenza season of the opposite side of the world. Usually, there are a few different strains of A or B running around but they usually select the ones that are most commonly seen during that season.
The biggest confusion here is that there is a mix up with two different types of vaccine. The influenza one covers for the influenza virus whilst the other one commonly used is pneumococcal vaccine (called prevenar), given for a prevention of bacterial infection that can lead to pneumonia or severe respiratory infections. The pneumococcal vaccine is usually only given to infants and elderly people (as these people are susceptible to these infections) whilst the rest of us don't really need to have this vaccine past childhood. The prevenar vaccine has twp types, one that covers 13 strains and the newer one (that is replacing the older one) that covers 25 of the most commonly seen strains of pneumococcal.
Next, I want to address the most commonly said reason why people don't want to have the vaccine. I have heard many times that people don't have the vaccine as when they have it they seem to get sick more frequently than if they don't. Or, they say that after getting the vaccine that they end up becoming sick within the week and therefore if they don't have it they won't get sick. Both of these are associative things that happen and I can assure you that the vaccine is not the causative reason (and I'll even explain why). Firstly, most of the "flu" infections you get are not influenza. People that have had influenza know what real influenza is. It is the kind of infection that you become very sick with (and sometimes need to be hospitalized). Severely sore joints, coughing/sneezing a lot, massive fatigue (to the point where you don't want to get out of bed to even go to the bathroom) and really high fevers. Most viral infections you get are a usual respiratory virus but not influenza (and therefore, the vaccine won't protect you against that at all). As for getting sick more frequently, this is hard to really explain why. Some are because they are more "socially" active (i mean doing more things in public, being exposed to more people than usual) and this increases the risk you would potentially catch something from someone. The last one I want to address is the one that people tend to get sick within a week after having had the vaccine given. This is NOT the vaccine causing this (yes, it can cause a little bit of low-grade fevers, headaches, etc). But actually getting sick from it is not what the vaccine does. The reason why is because you were probably already infected with a viral infection before you had the vaccine and it is now only becoming symptomatic. Most viral infections take a few days to "incubate". This means that you are infected, can spread it to others but you won't actually "feel" like you are sick. Most viruses have an incubation period of about a week to two weeks (depending upon the virus). Therefore, you probably have been infected up to a week prior to the vaccine being given to you.
Now, the biggest thing that gets on my nerve about the influenza vaccine is that people expect it to work miracles. This is a vaccine, it helps to reduce how sick you would be compared to if you didn't have the vaccine. This will NOT stop you from getting influenza at all. However, if you are unfortunate and do have it, it will make it much less severe than it otherwise would have been. For older people, this vaccine might prevent them from having to be admitted to hospital. They will still feel like crap having influenza, but not having to go to the hospital is a pretty big difference. Also, as I said that these are either trivalent or quadrivalent vaccine, it only covers for 3 or 4 of the most common ones that are floating around. You could catch one of the ones that weren't in the vaccine and in that case, you would still get pretty sick.
Now I read in some of the past posts about inflammation, not coughing and resting as much as possible otherwise you would make it worse. This is actually not true at all. Viruses have a specific progression and a usual duration of illness. We know what is likely to happen as you progress through with influenza. That is the reason why if you go to ED on day one of feeling unwell and are pretty sick, having difficulty breathing, not eating and/or dehydrated, we might err on the side of caution and look at admitting you (as we expect it to get worse). This is very different than someone who comes in after a week of having influenza and says that they are still feeling unwell, they just need a bit more time to get better but are over the worst of the infection. Coughing, sneezing, moving around and drinking lots of fluids are actually good things. Coughing helps to get rid of secretions in your lungs as well as sneezing gets rid of secretions/mucus in your nasal passage. Moving helps as it gets the blood flowing and this assist with the immune system to be able to attack the virus (or move to the lungs better to attack the virus there). Unfortunately, sneezing/coughing is also the way that influenza (and many other viral infections) are transmitted to other people. So, for this reason, it is best to steer clear from other people when you have these symptoms. Keep in mind though that as I mentioned earlier that you probably have already infected your co-workers/family as during the incubation period of 1-2 weeks you have already been spreading the virus around, you just weren't feeling unwell at the time.
Now onto the thing about why we need to keep changing the influenza vaccine. The reason why viruses continue to exist is because they are very good (and quick) at adapting and changing. There are two terms we use to describe this "antigenic drift" and "antigenic shift". Genetic drift is a random thing to occur. It is small changes in the RNA of influenza (its similar to DNA but a different thing) that means that our immune system isn't able to recognize the pattern like usual. This means that it can infect the person and cause serious illness as the immune system has to go through a lengthy process of learning the viruses "signature" (that is what we are aiming to do with the influenza vaccine). However, influenza is unique in that for some reason our immune system forgets about the "signature" relatively quickly. So that is why in a years time, even if you have had influenza, you can still end up getting it again.
What is more concerning is antigenic shift. This usually doesnt happen in humans, it more commonly occurs in other animals that can be infected by different types of influenza. For examle, the swine flu virus that people knew about occured because influenza in pigs was mixed with human influenza. This resulted in a completely different influenza that humans had not seen before (and cause serious lifethreatenting illness that had the potential to kill). Human influenza can infect other animals, but it tends not to make them sick as it doesnt work the same in them. Where the problem is if another animal is infected with a different influenza AND human infleunza at the same time. Imagine putting two similar viruses into a blender to mix it up and produce a new virus. That is what happens in antigenic shift. It is a complete shift in one (or more) areas of the code of a virus and this causes it to be very different from what we have seen before.
The combination of antigenic shift and antigenic drift is what makes developing a single vaccine for influenza so difficult (if not impossible) and this is why we have to develop one each year (and why we look at the flu season from the other side of the world).
Hopefully, this clarified up a little bit abput vaccines and influenza.
