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This Is Why COVID Vaccines Can Have Side Effects


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

clockMar 26 2021, 11:53 UTC
Person getting a vaccine

Person getting a vaccine. Image Credit: hedgehog94/

No medical intervention is risk-free, and researchers spend a great deal of time making sure that said risks are as rare as possible – and that they are worth it. Obviously, due to the pandemic, a great deal of attention has been given to the COVID-19 vaccines and their side effects.

About one in ten people experience side effects following the jab. This is mostly pain, redness, and swelling near the injection point, but people also experience headaches, chills, fever, nausea, and a general sense of tiredness. The side effects could be worse after the second jab than after the first.


But what’s the actual biological cause of the side effects? The team at Technology Networks has interviewed Professor Martin Michaelis and Dr Mark Wass of the University of Kent’s School of Biosciences to explain what goes on in your body after vaccination. This is part of their series Teach Me in 10.

“There are two main reasons why vaccines cause side effects. One is actually the response of the immune system and that is what we want,” Professor Michaelis said in the video. “The other one is that many people will know but might not have in mind when it comes to side effects or adverse events: the placebo effect. Because that can have positive but also negative consequences.”

Side-effects, though they are not welcome, have the silver lining of making it loud and clear that our immune system is active. The COVID-19 vaccines do not contain the virus SARS-CoV-2, which causes the disease. They just trick our body into producing a harmless part of the virus, which our immune system can then learn to recognize as dangerous. This allows our body to be more ready to fight in the case we actually get infected by SARS-CoV-2.

Michaelis talks about the innate immune response of our body and how it is the same for a wide range of diseases – this is why we can only find out if someone has COVID-19 for sure with a test. He also stresses that our body is not aware that there isn't any danger when it comes to the vaccine, so it responds like it would to another attack.


One particular example is vomiting. As a side effect, it was certainly on the rare side. In the AstraZeneca trials, a few people in every 100 experienced it. The vaccine is injected into your arm, and vomiting serves to purge your stomach of something dangerous, so it serves no purpose whatsoever in this case – but your body just goes with how evolution taught it to behave.

The second cause of side effects is the nocebo effect, similar to the placebo effect, but with negative consequences. Dr Wass explains that this comes from the expectation of side effects and then experiencing them.

You can check out the full interview below:


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