Booster jab side effects: Pfizer and Moderna booster side effects - and how the Covid booster vaccines differ

More than half of the Scottish population have had a booster vaccine so far - here are the side effects for booster jabs and what to watch out for.

With 52% of over 18-year-olds in Scotland having received their booster dose, as per the latest data from Public Health Scotland, Scotland’s booster vaccine rollout continues to be the fastest in the UK.

The rapid spread of the Omicron variant of coronavirus has led to people across the UK queueing up for booster vaccines as almost 90,000 positive Covid cases were reported in the UK on Thursday December 16.

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Booster jab side effects: Pfizer and Moderna booster side effects - and how the Covid booster vaccines differ (Image credit: Getty Images/troyanphotos via Canva Pro)
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But as more people are called to attend walk-in vaccination clinics in Scotland following Omicron’s rise, some people are also experiencing the mild side effects of the booster jab – which can differ depending on the dose and type of vaccine.

Currently, Moderna and Pfizer are the vaccines predominantly used in Scotland’s booster vaccine programme, with AstraZeneca also available to those with allergies to the mRNA vaccines.

Getting a booster vaccine is the best method we currently have of preventing the further spread of the Omicron variant, which has been shown to be able to evade one or two doses of a Covid vaccine more easily.

Here are the most common side effects to expect after your Moderna (also known as Spikevax) or Pfizer booster jab.

Members of the public queue for vaccinations on a vaccination bus at West College Scotland Clydebank Campus on December 17, 2021 in Glasgow. (Image credit: Jeff J Mitchell/Getty Images)


The Moderna booster vaccine, unlike the Pfizer booster, is a half dose of a single vaccine and administered under the name ‘Spikevax’.

The most common side effects felt by those receiving Spikevax, affecting more than one in ten people, were:

- Swelling/tenderness of the underarm glands on the same side as the injection site

- Headache

- Nausea

- Vomiting

- Muscle ache, joint aches, and stiffness

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- Pain or swelling at the injection site

- Feeling very tired

- Chills

- Fever

Other common side effects, according to the Medicines and Healthcare products Regulatory Agency (MHRA) include diarrhoea, rashes and redness or hives at the site of injection.

Less common reactions include rashes, redness, hives or temporary sloping of the face on one side (also known as Bell’s Palsy), but this and any facial swelling are rare reactions.

The UK Government advises that those experiencing any of these more rare side effects, along with any wheezing, shortness of breath, stomach pain or swelling of the lips, tongue or throat, chest pain or a fast-beating, fluttering or pounding heart, should seek urgent medical attention.

A number of very rare cases of heart inflammation such as myocarditis or pericarditis have been reported in those receiving Spikevax – occurring mostly in younger men and shortly after receiving their second dose of the Moderna jab.

But the MHRA states that the majority of these cases were mild and quickly resolved through simple treatments and rest.

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You can find out more about the Moderna booster jab and its side effects on the UK Government website.


Like Moderna booster vaccine, the Pfizer booster jab is considered to have many of the same, familiar side effects seen above.

These include pain, redness and swelling around the site of your infection to be expected along with chills, tiredness, headaches, fevers and diarrhoea.

Muscle and joint pain are also among the side effects seen in over one in every ten people.

More rare side effects of the Pfizer vaccine which are likely to affect one in 100 people include enlarged lymph nodes, insomnia, itching around the injection site, feeling unwell or having allergic reactions.

As per the results of the COV-Boost trial, fewer than 1 in 100 people had difficulty sleeping after receiving a Pfizer booster jab – with weakness of the muscles on one side of the face also observed in fewer than 1 in 1000 people.

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Myocarditis, inflammation of the heart muscle, was also identified by the UK Government as a further rare side effect of the Pfizer vaccine along with extensive swelling of the vaccinated limb and swelling of the face – which can occur in patients who have had facial dermatological fillers.

To find out more about the vaccines and how to watch out for allergic reactions, visit the NHS Inform website at:

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