Why do some people experience side effects from the Covid vaccine?

More than 2.5 million people have now received their second dose. (Picture: Getty)

The coronavirus vaccine rollout has already been successful in reducing infection rates, with the UK beating its daily record last week.

Some 844,285 vaccines were registered in the country on Saturday, which is around 1% of the population.

Those in the over-50s group are now being encouraged to book jabs, as centre closures are expected.

As with all vaccination programmes, a small number of people may have an adverse reaction – although rarely anything worse than a mild case of the virus itself. 

So, why do some people experience side effects from the Covid vaccine, while others don’t?

Why do some people react to the Covid vaccine?

Vaccinations are a way of stimulating an immune response to protect people against infections. 

Vaccinations are a way of stimulating an immune response. (Picture: Getty)

The Covid vaccines trigger a range of responses which are harmless, as the vaccine does not contain the part of the virus that is dangerous.

This response prepares the body’s white blood cells, allowing them to work against coronavirus more effectively in the future. 

Different types of vaccines work in different ways to offer protection, but with all types of vaccines, the body is left with a supply of ‘memory’ cells that will remember how to fight that virus.

Speaking to, NHS consultant Dr Sreedhar Krishna says that as a result of this process, a small proportion of people will experience a reaction to the Covid vaccine. 

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He says that ‘our immune system determines both our response to infection and to vaccination. Each person has a different make-up, with some people having a hyper-alert immune system.

‘These hyper-alert immune systems can overreact at the slightest threat, and could include people with asthma, eczema or hay fever.’

What are the most common types of reaction to the Covid vaccine?

Dr Krishna says that the most common types of reactions are:

  • A sore arm 
  • A headache 
  • Tiredness 
  • Chills
  • Mild flu-like symptoms.
Mild flu-like symptoms aren’t unusual after having the jab. (Picture: Getty)

These are often seen in people who have the flu jab or other types of vaccination, not just with the coronavirus vaccine. The vast majority of people will only experience mild symptoms for around one to four days.  

Reece Samani from Signature Pharmacy also spoke to, saying these reactions are to be expected. 

‘Both the Pfizer/BioNTech and AstraZeneca/Oxford vaccine have a number of common side effects which impact more than one in ten people’ he says. 

Dr Krishna’s advice for people worried about the potential for a reaction is to discuss the process with their doctor, as they will be able to give advice specifically tailored to their medical history. 

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