For some diseases, vaccinations are needed every few years, or even once in a lifetime. Why is the flu vaccine different?

The seasonal flu is already here. As every year, albeit with some delay this year, the public has been called to get vaccinated against the disease. The flu (influenza) is not the common cold, but a serious and dangerous viral disease that leads to hundreds of thousands of deaths around the world every year and many more hospitalizations. On average, one in a thousand people will die of the disease, mostly a person in one of the risk groups – children under five years of age, older adults, pregnant women, and people with a weakened immune system. However, each year, young and generally healthy people also because of flu and its complications. 
As a safe and effective preventive measure, we are obviously called to get the vaccine against this potentially dangerous disease, to protect ourselves and our loved ones. But why are we called to get vaccinated again and again, every single year? Other vaccines provide long-term protection – often life-long, as do the vaccines against polio and measles – one or a few doses are enough. Why is the flu vaccine special?
The answer here, like in many other instances in life, lies in evolution. The influenza viruses evolve much faster than other viruses we vaccinate against. A vaccination essentially teaches our immune system to identify a disease-causing agent so that it can fight and stop it before it does any harm. Many vaccines contain only parts of the disease-causing agent, which cannot initiate the disease on their own.
The immune system reacts to these particles, and within two weeks, it creates special proteins (antibodies) that can act specifically against them. The vaccine enables our immune system, upon subsequent contact with the entire virus, to recognize the particles the antibodies are specific to and launch an attack against the virus. In the influenza vaccine, the viral parts in the vaccine are proteins from the virus’s envelope.
Every time the virus replicates in the cells of its host’s body, random mutations occur in its genome, which could change the proteins on its envelope. These proteins change to such an extent each year that the immune system can no longer recognize them, in a process termed ‘antigenic drift’. These evolved viruses have an evolutionary advantage over other viruses that the immune system recognizes and eliminates; thus, they are able to reproduce, spread around the world, and lead to the following year’s flu. That is the reason why the old vaccine loses its effectivity and the protection it offers.
Moreover, sometimes one of influenza virus strains undergoes an extensive and rapid change, which leads to the sudden appearance of new viruses. This is a relatively rare occurrence, called ‘antigenic shift’, which can happen when different influenza viruses infect the same cell at the same time. The viral genetic material mixes and creates a new, hybrid, virus. This is how the Spanish influenza virus evolved – the virus that caused an epidemic claiming the lives of tens of millions of people between the years 1918 and 1919, as well as the swine flu of 2009 –originating in a mixture of avian, swine, and human influenza viruses, it killed hundreds of thousands of people, especially in places lacking access to vaccines and appropriate supportive healthcare.
Towards protecting people from new influenza strains, the World Health Organization (WHO) and local health organizations monitor the most common strains of influenza around the world. Twice each year, the WHO publishes its recommendations for the composition of the flu vaccine. Once – usually in February – for residents of the northern hemisphere, Israel included, and once, usually in September, for the southern hemisphere. The recommendations for each hemisphere are based on the viral strains that were most prevalent during the previous winter in the other hemisphere, and they are published six to eight months before the beginning of the winter, giving the pharmaceutical companies enough time to prepare the vaccine. As the composition of the vaccine is based on predictions, it is possible that new strains will appear after the vaccine was already manufactured. However, even in years in which the prediction for the strain composition of that winter is not completely accurate, the vaccine does provide partial protection, and those infected despite the vaccine usually develop a milder illness.
The influenza vaccine is composed of three or four strains; this year in Israel, only the four-strain vaccines are available. The current recommendation for the vaccine composition for the northern hemisphere was postponed by one month, to allow the WHO to follow changes in the circulation of one of the strains which was supposed to be included in the vaccine – leading to the delay in the supply of vaccines this winter. Eventually, this year’s vaccine is composed of two new strains and two strains from last year. When we get our annual flu shot, it is not another dose of the old vaccine, but a new one that will protect us from new strains of the influenza virus.