Why do we need to take the flu vaccine every year?

person getting vaccinated

Unlike the chickenpox vaccine and measles vaccine, we should get the flu vaccine every year. 

It is because that the influenza virus – the causative agent of the flu – changes over time, making the last year’s flu shot wholly or partially ineffective.

But how does the influenza virus changes so much over a year?

The influenza virus

The influenza virus is an RNA virus. And the RNA genome of the virus is segmented into eight different fragments instead of one continuous genome. Once inside our body, the influenza virus takes over our cells and manipulates our cell’s biochemical machinery to replicate the virus genome.

Replication is a process by which the virus copies its RNA genome to produce two identical sets. It allows the virus to make abundant copies of its genome to continue infection, thereby allowing its survival. The proofreading system in the replication ensures that the genome is copied exactly like the parent strand and rectifies the errors. Unlike DNA viruses, the influenza virus lacks the proofreading system. 

Let us compare the influenza virus to a basket (say green-basket-no .1 containing eight different beaded necklaces) and replication to beadwork. The beader beads eight necklaces exactly like the necklaces in the green-basket no.1 to put in the green-basket no.2. Then bead eight necklaces exactly like the necklaces in the green-basket no.2 to put in the green-basket no.3. And it goes on. The goal of the beadwork is to have all the green-baskets identical.


Without a proofreading system, this process is prone to error as sometimes one wrong bead might get into one or more necklaces. For example, that would contradict necklace no.1 in the green-basket no.3 and green-basket no.2.

And at the end of the beadwork, instead of identical green-baskets with exact copies of the eight necklaces, we might get similar green-baskets with modified copies. 

The modified necklaces in the basket are like the mutated genes of the virus. Because of the gene mutations, the influenza virus changes so much over time. This process is called antigenic drift. It occurs with both influenza A and the influenza B viruses.

And what if there are two subtypes of the influenza A virus co-infecting the same cell? It is like having the beadwork for the green-basket and red-basket in the same room. The odds to swap one or more of the necklaces from the green-basket with the red-basket are high. 

It is a type of genetic recombination between the two viruses. This process is called antigenic shift that can lead to pandemics. Antigenic shift occurs only with the influence A virus, not with the influenza B virus. 

Changes at the protein level

Genes code the proteins. The gene mutations affect the proteins. The hemagglutinin (HA) and neuraminidase (NA) are two surface proteins encoded by the influenza virus gene.  The HA and NA surface proteins of influenza viruses are antigens (meaning these are the proteins recognized by our immune system ) capable of triggering an immune response in our body.

The mutations in the influenza virus gene can cause changes in these surface proteins.

How does this affect the vaccine?

Vaccination is like a safety drill. The vaccine itself cannot cause disease, but it is sufficient to create a false alarm to conduct the safety drill. This drill trains our body to produce enough immune cells to fight the infection and produce memory immune cells to remember the intruder. And the next time we encounter the same virus, our body performs the necessary actions to defend against the virus.

But what if the intruder is in disguise? Our immune cells might fail to recognize the virus and take sudden action to defend against the virus even though our body is fully-equipped to fight the virus. Therefore, our immune system could not recognize or remember the influenza virus with altered antigens (HA and NA surface proteins).

How do the vaccine producers know what vaccine to make this year?

The influenza virus changes rapidly over time because of the lack of proofreading systems rendering the last year’s vaccine ineffective for the current year. How do the vaccine producers know what vaccine to make if the influenza virus is mutating over a year?

The flu vaccine production is a continuous global collaboration program under the World Health Organization (WHO). It is year-round surveillance for the influenza virus.

The hospitals and laboratories around the world send the influenza virus samples from the patients to five WHO Collaborating Influenza Research Centers, located in the following places:

• Atlanta, Georgia, USA (Centers for Disease Control and Prevention, CDC)

• London, United Kingdom (The Francis Crick Institute)

• Melbourne, Australia (Victoria Infectious Diseases Reference Laboratory)

• Tokyo, Japan (National Institute for Infectious Diseases)

• Beijing, China (National Institute for Viral Disease Control and Prevention)

The WHO reviews the surveillance and clinical studies with the five Collaborating Centers, other national laboratories, and academics. This meeting occurs twice a year – in February for the Northern Hemisphere and in September for the Southern Hemisphere. The meeting analyses which influenza viruses are currently in circulation, how fast the virus is spreading, how effective was the last year’s vaccine etc. And based on this meeting outcome World Health Organization (WHO) recommends the influenza strains to be included in the vaccine. But each country can decide the exact composition of the vaccine to use in their country.

The flu vaccine is made to best match the currently circulating strains of the influenza virus. The flu shot decreases the chances of getting flu and reduces the severity and complications of the illness if you get the flu. By taking the flu vaccine, you can also protect your dear ones who could not get the vaccine by limiting the overall number of cases and exposure. But the catch is you need to take the flu vaccine every year!


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