The short answer is that viruses mutate really, really fast. For living creatures, such as bacteria, and especially for higher animals, the risk of mutating fast is high–the wrong mutation will literally kill them. Consequently, when a bacterium or higher creature’s cell replicates, it proofreads the copied DNA to eliminate as many errors as possible. Viruses on the other hand, are a bit like Amazon–they try everything, just to see what works (not consciously, of course)–so they don’t have proofreaders. If there’s an error in the copy, it is passed on to all the offspring viruses.
The downside to this for humans is that this constantly changing virus makes our immune system play a never-ending game of catch-up. The virus you developed immunity to last year has now mutated into one you don’t have immunity to–or has stopped circulating. By the time you develop immunity to the new virus, it, too, will be gone and replaced by another virus.
This problem extends to vaccines. Vaccine development takes some time, so the vaccine makers have to start well in advance of the next flu season. The meetings on which viruses to include for the Northern Hemisphere vaccine happen in the previous February or March, so by the time we get the vaccine, it’s already more than six months old, and the viruses have had a chance to mutate. The bigger problem, though, is that predicting which viruses will go, well, viral, seems to be only somewhat more accurate than the weather. This isn’t the CDC/FDA/WHO’s fault. It’s thanks to the nature of viruses.
So why get the flu vaccine at all?
- If the flu viruses used to make the vaccine are even somewhat similar to the ones circulating that flu season, they may provide partial protection (your immune system will be able to recognize the virus somewhat, though not fully). So even if you get sick, it might be less severe.
- You might get complete protection against one of the several circulating strains of flu, and so only get sick once vs twice or more.
- The flu vaccine might work perfectly, and might protect you completely from the circulating viruses.
I’d also add that for anyone with a weak immune system (through age, chronic illness, or youth), the downsides of not getting the flu can be very, very high. Since many pharmacies offer highly discounted or free flu shots, it’s not typically a financial issue. Some people also worry about the wisdom of getting lots of shots if they might not really need them. This isn’t problematic to me–I strongly suspect that getting the flu is far more stressful to our bodies than getting an optional flu shot.
Useful timeline graph: https://www.who.int/csr/disease/swineflu/notes/h1n1_vaccine_20090806/en/