Explainer: Why do COVID-19 vaccines cause side effects?

Sam Wood
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How can the the vaccines be causing side effects?

A fraction of people experiences side effects after receiving their COVID-19 vaccinations. Professor Martin Michaelis and Dr Mark Wass of the School of Biosciences explain what might be the cause of this:

‘After receiving COVID-19 vaccinations more than one in ten individuals suffer from side effects similar to those of COVID-19. These including tenderness, warmth, pain, itching, or bruising at the injection site, tiredness, chills, joint pain/muscle ache, and nausea.

‘Less than 10% also experience redness, swelling, or a lump at the injection site, fever, vomiting, diarrhoea, and other infectious disease symptoms such as fever, cough, sore throat, and/or a runny nose.

‘Although this may feel like the real disease, it is actually the response of the immune system to the vaccine. None of the available COVID-19 vaccines contains SARS-CoV-2, the coronavirus that causes COVID-19. They use different strategies to present just parts of the virus to the immune system and they do not cause an infectious disease.

‘The reason why these symptoms may feel similar to the disease the vaccine protects us from is that both the disease and the vaccine induce a similar immune response. This vaccine-induced immune response will then protect you from COVID-19 in the future.

‘Many infectious disease symptoms are not caused by the pathogen but by our own immune response, which is why many symptoms are generic across many diseases. This is also why it is impossible to discriminate with certainty between infectious diseases without a test.

‘Notably, vaccine side effects may also be caused by the “nocebo effect”, a polarised alternative of the famous “placebo effect”. Whilst the placebo effect is associated with clinical improvements seemingly caused by psychological effort from the patient, they can also induce negative responses, the “nocebo effect”.

‘Negative responses are known to have resulted in the discontinuation of therapies in up to 25% of patients in placebo arms of clinical trials. Sometimes, side effects are higher in the placebo group than in the group receiving the active treatment.

‘Similar levels of negative responses are reported in the treatment and placebo arms of COVID-19 vaccine trials. So, for some vaccinated individuals the knowledge that they have been vaccinated may be sufficient to drive side effects.

‘Ultimately, side effects associated with COVID-19 vaccinations are not a sign of the disease. COVID-19 vaccines are not capable of infecting anyone with SARS-CoV-2, the coronavirus causing COVID-19. Negative results are likely caused by the immune response to the vaccine, or the knowledge that we have been vaccinated acting as a “nocebo”.

Professor Martin Michaelis and Dr Mark Wass, School of Biosciences

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L-R: Prof Martin Michaelis, Dr Mark Wass