Short term side effects following Pfizer and AstraZeneca vaccines: data from first 7 million UK jabs

The clinical trials of all currently used vaccines studied relatively small numbers of people. As volunteers they were also healthier and younger than the general population.  Now, after millions of doses of the Pfizer and AstraZeneca vaccines have been given in the UK, we have much more data which apply especially to older people and other groups vulnerable to the consequences of Covid-19.  I think we all need this information both for our own reassurance and to counter concerns of others.

Source of the new data

  • In my previous blog* (posted on my birthday 4th December!) I outlined the different sources of data on side effects from vaccines following their introduction
  • The most rapid source of data is based on individuals and their health care professionals reporting to a centralised database a suspected side effect


  • The main disadvantages of this system are threefold:
    • It is impossible to calculate the actual frequency of any reported medical condition after vaccination.
    • There will be substantial under-reporting but the completeness is likely to be greater though for novel and/or severe problems
    • To make sense of the number of reports of serious disorders, we also need to know how frequently such cases may arise in the general (ie unvaccinated) population. 
    • The fact that even an unusual disorder follows a few days after vaccination does not prove cause and effect 
  • With these cautions, I consider the recent data produced by the UK regulator (MHRA) on possible side effects reported from the first 5.6 million doses of the Pfizer vaccine and 1.5 million doses of AstraZeneca vaccine 
  • In total MHRA received around 23,000 reports of adverse event, equivalent to 3/1000 jabs

Expected side effects

  • We already knew from the clinical trials that there were a number of very common, short-lived, side effects and a high proportion of these 23,000 reports were for these same symptoms
  • From the MHRA reports can be added other common short-lived side effects: 
    • Numbness/pins and needles in the arm and/or hand
    • Rapid heart rate/palpitations
    • Diarrhoea
    • Shaking
  • The occurrence of all these common side effects could be considered ‘good news’!
  • Remember the vaccines involve injecting into muscle a relatively large quantity of foreign material 
    • We want there to be an immediate response from the body’s first line, general immune defence mechanisms
    • These are referred to as our innate immunity, are nothing to do with antibodies and are not specific to any vaccine or indeed any infection
  • If you don’t have any side effects, that doesn’t mean the vaccine hasn’t worked for you
  • The other major reason of course for collecting data about these side effects is that vaccine centres/patient information websites etc can draw on these to provide reassurance to people that their symptoms 
    • Are not unusual
    • Do get better
    • Are a sign that the vaccine is working
  • There are conflicting reports as to whether side effects are more or less likely with a second dose.  This would be useful public information but there is not a definitive answer yet

Anaphylaxis/severe allergic events: the concern This has been the major concern for many people who have had severe allergic reactions to vaccines, other medicines as well as other chemicals

  • The concern has been more about what else is in the vaccine as opposed to the actual active bit
  • None of the vaccines contain egg albumen
  • The Pfizer (and Moderna) vaccines do contain polyethylene glycol (PEG), a chemical which is also widely used in food preparation
  • PEG enhances the immune response to the active component in mRNA vaccines 
  • Allergy to PEG does occur but is rare and people who have experienced this in the past are advised to avoid the Pfizer vaccine.  
  • The AstraZeneca vaccine does not contain PEG but does contain a similar compound, Polysorbate 80, which is probably less allergenic than PEG
  • National authorities only recommend not being vaccinated if there has been an issue with a constituent like PEG
  • Allergies to other chemicals/foods/drugs are not a contra-indication to being vaccinated 

Anaphylaxis and severe allergic events: the data

  • Obviously we don’t know how many people have refused vaccines because of concerns, whether appropriate or not, because of past allergy history
  • There have been only a tiny number of cases of anaphylaxis reported in the UK
  • All these patients recovered

*In the UK because of the issue with PEG, people who are vaccinated with the Pfizer vaccine are asked to wait for 15 minutes before leaving.  The risk from polysorbate is considered much lower

Conclusion: Severe allergy to vaccines is only an exceptionally rare complication

Severe neurological complications

  • There was widespread publicity in September to a case of the rare spinal disease, transverse myelitis, in a participant in the AstraZeneca trial
  • This led to a pausing of the trial in UK and (for a much longer period) in the USA) 
  • The good news  from the MHRA data is the extreme rarity of any serious neurological complication
  • Just one case of transverse myelitis each in the Pfizer and AstraZeneca vaccines
  • And of course those cases may be completely unrelated to the vaccine 
  • The other serious neurological disorder, which was reported in one individual during the early clinical trials, was Guillain-Barré syndrome (GBS): a rare disease which causes progressive muscle weakness because of an immune reaction, possibly to a virus
  • Indeed GBS is a theoretical complication from Covid-19 infection itself
  • The good news again is that only one further case has been reported with the Pfizer vaccine and none from the AstraZeneca vaccine
  • There have also been no other reports of other serious, possibly immune-related neurological disorders such as multiple sclerosis 
  • Conclusion: The worries about severe neurological complications are at this stage unfounded

Any unexpected side effects?

  • The only side effect reported in any significant number is Bell’s palsy (see picture at the top of this post)
    • This disorder is a paralysis of the facial muscles on one side
    • Can be a complication of a viral infection 
    • It does get better and most people are fully recovered by 6 months
    • Is treated with steroids
  •  In the UK dataset  there have been:
    • 69 cases following Pfizer (13/million)
    • 6 cases following AstraZeneca (4/million)
    • But around 200/million people per year in the general population get this condition
  • Conclusion: Despite the publicity the number of these cases is no higher than the general population incidence 

Why I find these data important

  • I have been giving a number of online talks to community groups about Covid-19 in general and, more recently, vaccines in particular.  Two weeks ago I came across my first ‘anti-vaxxer’ who asked:

How could I guarantee the vaccines were 100% safe (especially for a young non-vulnerable person such as myself?)”

  • I think my answer was a bit weak as I emphasised the much greater and known problems from Covid-19 itself
  • If I was asked this now my answer would be:

“I am confident that there is  a much bigger risk of having an accident in the car on the way to vaccination than any serious health risk from the jab itself”!

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8 replies on “Short term side effects following Pfizer and AstraZeneca vaccines: data from first 7 million UK jabs”

Very informative as usual thank you Alan. Regarding anaphylaxis and severe allergic events, in working out the rate, you used the number of people vaccinated as the denominator. But not all people are equally susceptible to these types of events. Would it be possible to estimate a rate using as the denominator, the number of people who have a history of severe allergic reactions, e.g. to nuts, egg etc? I think this would be helpful to people in that category who are hesitant.


Interesting suggestion but operationally difficult. The denominator MHRA has is only the number of vaccinations. It is also very difficult to know what constitutes a positive history of allergies and I am not aware that there is a register of who these people are. In theory the NHS vaccination centre could collect these data, and then follow up those subjects particularly. All the expert allergists do not feel that allergies to other things like egg, peanuts etc is an issue for these vaccines. The Centres for Disease Control, the USA’s much better equivalent of Public Health England are very positive that such allergies should not be a barrier to Covid vaccine (see )


One interesting anecdotal effect – it seems that those who have previously been infected with the coronavirus are much more likely to feel unwell after vaccination with either the Pfizer or the Oxford vaccine. Thankfully this only seems to last for up to 48 hours.


That’s certainly what the limited data suggests. Which is interesting as the assumption is for such people their previous illness is like their first vaccine dose. One issue is that AZ said in their trials that side effects were less common after the second dose. Anecdotal experience is suggesting the opposite. Either way these do resolve. My anxiety though is younger people will see these data, and if infection rates fall, then their willingness to be vaccinated will also drop


Alan, a belated question on this topic: do we know why the short-term side effects are (apparently) more pronounced the younger the person? My 27-year-old daughter was off work (in a vaccination centre!) for two days with flu-like symptoms, I had a very mild headache for 12 hours (have had far worse hangovers).
Thanks for all your blogging – it really is wonderfully informative.


The early data from AZ did actually suggest greater side effects in younger people, so probably is true. My theory is that the side effects are due to the body’s early non-specific (ie not antibody) immune response to the vaccine. Younger people have a stronger response.


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