Covid: What are the side effects of the AstraZeneca and Pfizer vaccines and why do only some people get them?

Most people suffer no side effects from the vaccine. Credit: PA

The government has urged anyone who has had any side effects from a Covid vaccine to come forward as a precaution despite medical regulators confirming their safety yesterday - but why do some people suffer more from the jab than others?

The recent controversy around the Oxford/AstraZeneca jab was put to an end yesterday when both the UK's Medicines and Healthcare products Regulatory Agency (MHRA) and European Medical Agency (EMA) confirmed there was no connection between the jab and blood clots.

Despite this Dr June Raine, chief executive of the MHRA said while they continued to investigate the issue, anyone who suffered a headache that lasts more than four days after the vaccination, or bruising beyond the area where the jab was injected, to come forward as a precaution.

Thousands of people have already come forward reporting side effects from both the Pfizer and AstraZeneca vaccines, but the vast majority of these have been for minor ailments like tiredness and soreness.

What are the most common side effects?All vaccines, as with medicines in general, can have some form of side effect.

Credit: PA

For both vaccines, the overwhelming majority of reports relate to sore arms at the site of the injection, and flu-like symptoms such as headache, chills, tiredness, nausea, fever, dizziness, weakness, aching muscles, and rapid heartbeat.

These are typical reactions to all types of vaccines and tend to resolve within a day or two.

The government says side effects levels are broadly similar across all age groups and genders, with them being slightly more common in younger adults (which was also reported in the clinical trials).

The ZOE symptom study has found there is an increased chance of suffering from side effects if you've already had Covid, with a third of people who had already had the disease reporting feeling ill after, compared to 19% for people who haven't.

They also found women were likely to report a side effect (19%) compared to men (13%).

What about uncommon side effects and allergic reactions?

The MHRA said data showed that severe allergic reactions to the Pfizer/BioNTech vaccine were very rare (less than one in 10,000 people receiving the jab), with a rate of one to two cases per 100,000 doses administered.

Overall, the MHRA received 101 reports of severe allergic reactions – associated with anaphylaxis or anaphylactoid reactions – for the Pfizer jab, and 13 for the Oxford vaccine. All the people recovered, as far as the MHRA are aware.

The regulator said anaphylaxis can be a very rare side-effect to many types of vaccines.

The regulator also received 69 reports of facial paralysis or weakness with the Pfizer/BioNTech vaccine and six for the AstraZeneca jab.

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It should be noted that the vast majority of these reports are voluntary, and the MHRA asks people to report any illness after getting the vaccine.

With the majority of people who have been vaccinated being elderly and vulnerable, it is often difficult to discern if the illness was caused by the vaccine or it was something that would have happened without the jab.

How do you report a side effect?

The MHRA runs a Yellow Card system for reporting side effects for all medicines in the UK.

You can report the side effect on their website or the Yellow Card app.

All you will need to do is tell them which vaccine you were given, your personal details and the side effects you felt.


As of 7 March 2021, for the UK, 35,325 Yellow Cards have been reported for the Pfizer/BioNTech, 61,304 have been reported for the Oxford University/AstraZeneca vaccine, and 281 have been reported where the brand of the vaccine was not specified.For both vaccines the overall reporting rate is around three to six Yellow Cards per 1,000 doses administered.

What about blood clots?

After several European countries suspended the use of the AstraZeneca jab over reports some people suffered blood clots after getting the jab regulators began an investigation.

They found for the most common types of blood clots there was no relation at all with the jab.

Several cases of an exceptionally rare blood clot were reported which was investigated by the regulators.

After finishing its investigation the EMA has said it “cannot rule out definitively” a link between “a small number of cases of rare and unusual but very serious blood clotting disorders” and the vaccine, though investigations were ongoing.

Science Editor Tom Clarke discusses the investigation into blood clots

The type of clot – cerebral venous sinus thrombosis (CVST) – prevents blood from draining out of the brain.The MHRA said it was looking at the reports but stressed the events were “extremely rare” and there was a possibility they could have been caused by Covid itself.

It said the cases represented a less than one-in-a-million chance of suffering this type of clot among those who have been vaccinated, while the risk of dying from Covid aged 40 to 49 is one in 1,000.

Five men in the UK have suffered from the blood clot after having the AstraZeneca coronavirus vaccine, though no causal link with the jab has been established.

The men, aged 19 to 59, have experienced a specific type of blood clot in the brain together with low blood platelet count. One of the five has since died.

ITV Science Editor Tom Clarke said Covid was known to also cause this type of blood clot.

He added the leading cause of this type of blood clot in the UK was the contraceptive pill, which can increase a woman's chance of suffering cerebral venous sinus thrombosis by eightfold.