Vaccines work by showing the body a microbial mugshot of a tiny perpetrator (such as a virus or bacterium) that is a threat to your body. This triggers an immunological fire drill against the invader, enabling your body to react much faster and better to a real attack. It’s this immune response that can cause some side effects.


Because everyone’s immune system differs depending on multiple factors including genetics, age, biological sex, existing illnesses, and even our microbiome, it responds differently to vaccines. For example, an aged immune system has a good memory for dealing with pathogens it’s seen before, but it isn’t as good at dealing with novel diseases like COVID-19.

Similarly, biological females have two X chromosomes compared to one for biological males. Many genes on the X chromosome are known to play a role in immunity, and marked differences between the sexes are being increasingly seen, with biological females having stronger immune responses. This translates to more side effects in women and fewer in the elderly.

Side effects may be linked to the use of adjuvants. When safely added to some vaccines, these materials (such as aluminium), can further fire up the immune system when a bigger response is needed than the microbial mugshot alone can provide.

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If you feel headachy, fatigued and even feverish post-vaccine, it’s a sign your immune system is working. These physical signs of the body’s response to vaccination even have a name: reactogenicity. But don’t blame the vaccine’s microbial elements entirely. Other factors include administration – the angle, needle size, location and speed of vaccination can all play a role.

If your arm hurts, remember that not only is your immune system reacting and releasing chemicals to stimulate an immune response, but you’ve also just been stabbed in the arm with a needle!

Asked by: Amelia Calderbank, Runcorn

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Stephanie Organ
Stephanie OrganScience editor and writer

Stephanie holds a Bachelor’s in Biological Sciences and a Master’s in Science Communication, and has an interest in health and wellbeing.