Why do some people react to the Covid vaccine? Side effects explained - and why not everyone gets them

The Covid vaccination programme is continuing to be rolled out across the UK, with nearly 36 million people having received at least one dose of a coronavirus jab.

Although many will experience little or no side effects after the Covid vaccine, there’s a chance that some may react to the jab.

But why do some people react to the vaccine and what are the side effects?

Here’s what you need to know.

What are the side effects of the Covid vaccine?

Most side effects of the Covid-19 vaccine are mild and should not last longer than a week, according to the NHS.

You can report any suspected side effect using the Coronavirus (COVID-19) Yellow Card safety scheme.

The Coronavirus Yellow Card website also says: “Vaccines are the most effective way to prevent infectious diseases and they save millions of lives worldwide.

“Like all medicines, vaccines can cause side effects. Most of these are mild and short-term, and not everyone gets them.”

Side effects can include:

- a sore arm where the needle went in

- feeling tired

- a headache

- feeling achy

- feeling or being sick

You may also get a high temperature or feel hot or shivery one or two days after having your vaccination.

Why do some people react to the Covid vaccine?

The coronavirus vaccine works by tricking the body into believing it has a virus so it can build an immune defence against it, which is what’s happening when you experience muscle aches, arm soreness or any other symptom of inflammation after your jab.

This response prepares the body’s white blood cells and enables them to work effectively against Covid-19 in the future.

Although different types of vaccines work in different ways in order to offer protection, all types of vaccines leave the body with a supply of ‘memory’ cells.

These memory cells will remember how to fight that particular virus.

Not everyone will get side effects after a Covid jab, whereas some will have a worse reaction than others, but it’s not yet known why this is the case.

However, if you’ve ever had a serious allergic reaction in the past then you should tell healthcare staff this before you receive the Covid vaccine.

You should not have the Covid-19 jab if you have ever had a serious allergic reaction (including anaphylaxis) to:

- a previous dose of the same vaccine

- any of the ingredients in the vaccine

Although serious allergic reactions are rare, if you do have a reaction to the vaccine then it usually happens in minutes.

Staff giving the vaccine are trained to deal with allergic reactions and treat them immediately.

The NHS website said: “So far, millions of people have been given a Covid-19 vaccine and reports of serious side effects, such as allergic reactions or clotting problems, have been very rare.”