Why There's a New Flu Shot Every Year

As the temperature drops and you’re dodging every stranger on the sidewalk who’s sneezing, coughing or clutching a tissue, you know that flu season has arrived. It isn’t an enjoyable time of the year since you worry that your child returning home from school with a runny nose has succumbed to the virus and will need time off to recover.

We hear one question quite often at First Pediatric Care Center, PA: Why do I need a flu shot every year? No other vaccine needs to be repeated on an annual basis, so what’s different about influenza?

The influenza virus changes over time.

Flu is a complicated and unpredictable virus that changes over time, unlike diseases like measles and Rubella that remain the same. This is because of two processes known as antigen drift and antigen shift that cause the virus to change its appearance, which means that from year-to-year, your immune system can no longer recognize and protect you from it.

The virus that causes influenza is self-replicating and copies genetic information from generation-to-generation. Sometimes errors are made when copying the information to create new cells. These mistakes can accumulate over time until the proteins of the virus have changed so much that it’s evolved into a new strain. This is the definition of antigen drift.

Antigen shift is a process that only occurs in Type A influenza, which has eight segments of genetic material. If a cell is infected by two different strains of the virus at the same time, it is possible for entire segments to be “shifted” and swapped between strains. This then creates an entirely new never-before-seen strain from which you have no natural immunity.  

The circulating flu virus is different every year.

Even though you may have had influenza last year and your immune system has built up a defense against it, because of the drift and shift processes the virus has traveled since then, it’s unlikely that you’ll be re-infected by the exact same strain of the disease. Over time, your immunity to the virus will naturally drop, so doctors recommend an annual vaccine to ensure you’re at optimal protection.

The World Health Organization (WHO) meets in February of each year to predict which strains are likely to hit the U.S. in the upcoming fall and winter. Patterns that have been seen in the southern hemisphere are used to inform the flu vaccine strategy as these tend to circulate around the world thanks to transatlantic travel.

Based on predictions made by the WHO, two influenza vaccines are developed: the trivalent vaccine contains three strains of the virus and the quadrivalent vaccine contains four strains. Which vaccine is right for your child depends on their age, natural immunity, and any conditions they may have, like diabetes, which can affect their immune response.

The flu vaccine can protect more than just your child.

It’s unusual for children to be severely affected by influenza, so many parents feel it’s unnecessary for them to have the vaccine. However, the CDC recommends that everyone over six months of age receive the vaccine to create what is known as “universal protection.”

That’s because children can carry the highly contagious virus and spread it to older and more vulnerable people who have a less robust immune system or who may not be able to have the flu vaccine to protect themselves. It isn’t unusual for parents to be struck down with flu while their children carry on as normal.


To boost your child’s immune system against influenza this year and protect your entire family from the virus, book online to schedule their vaccine with Margaret Lubega, MD.

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