While you might sometimes get sick after having a flu shot, it’s a myth that having a flu shot can give you the flu.
Influenza is a moving target for vaccines. Each year, up to four different strains circulate, and they are constantly evolving to escape our immune system.
So rather than childhood jabs giving long lasting immunity, we need annual influenza shots to provide optimal protection against influenza.
A quick history of the Influenza vaccine
Influenza vaccines were first developed in the 1930s and 1940s, starting with the isolation of the influenza virus.
Back then, we learned there were many different influenza strains. To be effective, early research showed the vaccine needed to be matched to the circulating strains, and to be able to stimulate a response from the immune system.
The process to produce modern influenza vaccines now occurs on a much more refined and industrial scale. Hundreds of thousands of influenza viruses are collected by hundreds of national influenza centres around the world.
From these, four strains are chosen for the annual flu vaccine, based on the viruses that are circulating at that time, how well the vaccines activate the immune system, how the strains are evolving, and the effectiveness of previous vaccines.
Most modern vaccines are made by growing large quantities of live virus – mostly in chicken eggs or less commonly animal cells – which are then purified, deactivated and split into smaller components. These vaccines are inactive and cannot replicate.
So why do you sometimes get sick after a flu shot?
There are several reasons why you might feel a bit off after getting your flu shot.
First, your flu shot only protects you against influenza and not other respiratory illness which might causes similar cold or flu symptoms.
Second, stimulating the immune system can result in symptoms similar to that of influenza, although much milder and short-lived. These include local inflammation (redness, pain or swelling at the site of the vaccine) and more general symptoms (fever, aches and pains, tiredness).
The flu vaccine also doesn’t “kick in” for two weeks after vaccine administration. In some people, particularly those who are older and those who have weakened immune systems, antibody production is not as strong, and the level of protection is lower.
Despite this, studies have consistently shown that vaccinated people are less likely to get the flu than those who aren’t vaccinated.