An investigation into whether the Oxford-AstraZeneca Covid vaccine is linked to blood clots has led many people to question the side effects of the Pfizer-BioNTech jab.
Fears over clotting resulted in several European countries halting the use of the AstraZeneca vaccine.
But the European Medicines Agency (EMA) confirmed that the jab was “safe and effective” to use on Thursday 18 March following its analysis of reports of blood clots in a small number of recipients.
As the Pfizer jab was the first to be rolled out in the UK, it has previously come under intense scrutiny over its safety and efficacy.
So, what are the Pfizer vaccine side effects, is it safe - and how does it compare to AstraZeneca?
Here is everything you need to know.
What are the Pfizer-BioNTech vaccine side effects?
In clinical trials, scientists found that the Pfizer vaccine had no serious side effects after it was tested on 43,500 people.
Any side effects that were experienced by patients were mild and similar to those associated with the flu jab.
Volunteers who received it compared their side effects to a “severe hangover”, according to a report.
Some said they could tell they had received the vaccine, and not a placebo, due to experiencing headaches and muscle aches.
According to the Vaccine Knowledge Group, the most common side effect experienced after patients were given this jab was arm pain, with 92 per cent of people experiencing a sore limb after their first dose.
Meanwhile, half of people had a headache, while 42 per cent experienced fatigue.
The least common side effects after this vaccine was administered were both fever and joint pain.
In December last year, there were widespread concerns after two NHS workers in the UK had an allergic reaction to the jab.
Both people, who have since recovered, have a history of serious allergies and need to carry adrenaline pens around with them.
Reactions to vaccines are not uncommon, and can happen with others like the annual flu jab.
Is the Pfizer vaccine safe?
No serious safety concerns emerged when the Pfizer vaccine was tested on thousands of individuals.
Before a large number of people were injected, the manufacturer ran smaller clinical trials in May 2020 which were used to detect any initial safety issues.
Four versions of the vaccine were tested, and the one that produced the fewest cases of mild and moderate side effects - such as fever and fatigue - was advanced to the next stage.
If there were dangerous side effects after taking the vaccine, these would have become apparent then.
Approval is only given in the UK if the medicines regulator is happy that a vaccine is safe and effective.
Scientists are still unsure about the long term effects of the Covid vaccines, but most side effects occur very quickly and don’t last for long.
There have been very few cases where problems have developed later.
If any issues do arise, they will be continuously monitored under the Medicines and Healthcare products Regulatory Agency’s yellow card scheme.
How do the Pfizer side effects compare to AstraZeneca’s?
Similarly, the Oxford-AstraZeneca vaccine was shown to be safe in trials, with only minor side effects observed.
The EMA said the benefits of the jab outweigh the risks, and it is not linked to an overall risk of blood clots.
Like the Pfizer jab, mild side effects after receiving AstraZeneca’s offering included injection-site pain, mild fever, muscle ache and headache, according to findings from the phase two safety trial which were published in The Lancet in November 2020.
The Vaccine Knowledge Group states that the most common side effect associated with this jab is fatigue, with 70 per cent of patients experiencing this.
That is closely followed by arm pain at 67 per cent, and just over half of patients experienced chills afterwards.
The least common side effect was fever at 18 per cent.
These short-term side effects are common with many vaccines and more serious, adverse effects have not been reported.
The side effects were also less common in adults aged 56 and older, compared to younger adults.