Q&A: Covid vaccine side effects – What are they, who gets them and why?

What are the most common side effects from the Covid vaccines?
Most side effects from the Pfizer/BioNTech and AstraZeneca/Oxford Covid vaccines, the two most common jabs given in Ireland so far, are mild and short-lived. These include soreness where the jab was given, feeling tired or achy and headaches. Uncommon side effects include having swollen lymph nodes.

Are blood clots a side effect of the vaccines?
There have been some cases of blood clots – a rare type on the brain called cerebral venous sinus thrombosis – or low platelets among recipients of both the AstraZeneca and Pfizer vaccines. That's why the National Immunisation Advisory Committee advised the Government this week that the AstraZeneca vaccine should not be given to patients under 60.

The benefits still far outweigh the risks for a 60-year-old, who is 85 times more likely to die from Covid than to have a clotting event of any kind after vaccination, according to Prof Karina Butler, who chairs Niac. But the benefit is lower, and the risk therefore higher, in young people: 45- to 49-year-olds are 12 times more likely to die from Covid as to have a clotting event, and 20- to 34-year-olds are only two times more likely.

The European Medicines Agency emphasises that, overall, the risk of clotting is small and not definitively linked to the AstraZeneca jab, and that it is therefore important that the vaccine continues to be used – a message repeated by the World Health Organisation. "This vaccine is a safe and effective option to protect citizens against Covid-19," Emer Cooke, the EU agency's executive director, said.

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Why do the common side effects occur?
"The sore arm can be either due to the trauma of the needle in the muscle, or local inflammation in the muscle probably because of the chemicals in the injection," says Prof Robert Read, head of clinical and experimental sciences within medicine at the University of Southampton.

“The other common side effects – the muscle aches, flu-like illness and fatigue – are probably due to generalised activation of the immune system caused by the vaccine. What this means is that the white blood cells that are stimulated by the vaccine to make antibodies themselves have to secrete chemicals called cytokines, interferons and chemokines, which function to send messages from cell to cell to become activated.”

Read says that for some people the process is without symptoms, but for others it generates these common side effects.

Are the common symptoms worse for the first or second shot?
For the AstraZeneca jab, it seems such side effects are more common after the first shot. "When compared with the first dose, adverse reactions reported after the second dose were milder and reported less frequently," the UK's Medicines and Healthcare products Regulatory Agency notes.

But for the Pfizer jab the reverse appears to be true. “Side effects such as fever, chills, tiredness and headache throughout the body were more common after the second dose of the vaccine,” according to the US Centers for Disease Control.

Read says: “We are a little baffled about this, but it may be due to the fact that the AstraZeneca vaccine has an adenovirus vector, which stimulates the immune system strongly in the first dose and less strongly in the second.”

Are people who have had Covid more likely to get these side effects?
Some data suggests this may be the case. According to the Zoe Covid symptom study released last month, and looking only at the Pfizer jab, about a third of vaccine recipients who had previously had Covid reported having a whole-body side effect (such as chills), compared with 19 per cent of those who had not had Covid.

Read says: “Usually if you have experienced a natural infection – in this case with the coronavirus – and are then challenged with something that looks quite similar – in this case the vaccine – an efficient immune system responds very quickly to the second challenge.”

Are there other groups who are more likely to experience side effects?
The data suggests that side effects are more common among younger recipients. "We have seen that older people are getting much milder side effects. It's not obvious why this should be – perhaps younger people have a much more robust reaction to the priming by the first dose," says Read.

Does a stronger set of side effects suggest you will have better protection against Covid?
Not necessarily. "All the evidence we have is that there isn't a correlation," says Simon Kroll, professor of paediatrics and molecular infectious diseases at Imperial College London. "That evidence best comes from looking at the responses from older people and younger people, because the evidence is that the vaccines are very effective right across the age range, but the side-effect profile is weighted towards younger people." – Guardian, with additional reporting