A new study suggests more than a third of people who receive their first dose of the coronavirus vaccine report some side effects - and a more frequent reaction in people who have had Covid "could be good news".
Most of the side effects with the first dose were mild, and included pain or swelling around the site of the injection.
The data from the study suggests people who have previously had Covid-19 are almost twice as likely to experience one or more mild whole body (systemic) side effects, compared to those who have not had the virus (33% vs 19%) from a Pfizer/BioNTech vaccine dose.
According to the latest data from the Zoe Covid Symptom Study app, the most common mild whole body (systemic) side effects were fatigue (9%), headache (8%) and chills or shivers (4%).
Tim Spector, lead scientist on the study and professor of genetic epidemiology at King’s College London, said: "This data set is a unique look at those who have been vaccinated in the real world outside trials, and so far the post-vaccine effects we see are mild and in the minority of people.
"It’s interesting to see that those with previous Covid are more likely to experience these mild after effects than naive subjects.
“This could be good news, as a larger response like this suggests that those getting a first dose after having had Covid are generating a stronger immune reaction and may get greater protection from just a single shot of the vaccine.”
Most mild whole body (systemic) side effects appeared in the first two days after the vaccination and only 3% of people have any after effects beyond three days.
The figures, based on a sub-sample of almost 40,000 vaccine doses, suggest that 37% of people experienced some local side effects after the first dose, and 45% after the second.
While 14% of people reported at least one whole body side effect within seven days from the first dose compared to 22% from the second dose, possibly indicating a stronger immune response after the second dose.
The data found that 13% of vaccinated men, and 19% of vaccinated women, reported at least one systemic side effect within seven days.
Under 55s were more likely to experience whole body side effects than those over 55s (21% vs 14%).
Covid-19 vaccines work by using a harmless version or component of the coronavirus to train the immune system, so when the virus is encountered the body is able to fight it off.
This response can feel like some of the symptoms when the body is fighting off a real infection, including headaches, fever, chills or shivers, tiredness (fatigue), muscle or joint pains, diarrhoea and feeling sick (nausea).
Experts say a stronger response may indicate evidence of an increased immune response.
This current data is for the Pfizer/BioNTech vaccine and most of those analysed were healthcare workers.
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