More than a third of people who receive their first dose of the coronavirus vaccine report some side effects, new data suggests.

Most of the side effects were mild, and included pain or swelling around the site of the injection.

The data 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 per cent vs 19 per cent) 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 per cent), headache (8 per cent) and chills or shivers (4 per cent).

Vaccines

Vaccines

Most mild whole body (systemic) side effects appeared in the first two days after the vaccination and only 3 per cent 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 per cent of people experienced some local side effects after the first dose, and 45 per cent after the second.

A total of 14 per cent of people reported at least one whole body side effect within seven days from the first dose compared to 22 per cent from the second dose, possibly indicating a stronger immune response after the second dose.

The data found that 13 per cent of vaccinated men, and 19 per cent 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 per cent vs 14 per cent).

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.

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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.

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.”

Medical trials so far suggest the vaccines are safe and effective. Doctors say people with a history of significant allergic reactions, rather than general allergies, should not have the Pfizer-BioNTech vaccine or the similar Moderna vaccine, however, because of a small number of allergic reactions needing treatment.

In the UK, people can report concerns to the Medicines and Healthcare products Regulatory Agency's Yellow Card scheme.

Dr Anna Goodman, at Guy's and St Thomas' Hospital, in London, who has been running trials with other coronavirus vaccines, from Oxford-AstraZeneca and Novavax, told the BBC that side effects may be unpleasant but suggested the immune system was responding to the vaccine.

But people still needed to follow social-distancing rules, because protection may not be 100%.

"Because you have more fever, that doesn't mean you are more immune," she said.

"You can't presume it to. Although, it does suggest that your immune system is, of course, doing something."

And people could take paracetamol for mild after-effects, such as a sore arm, she said.

Lead Zoe app researcher Prof Tim Spector told the BBC: "Generally, most people should be reassured by this data."

He said anyone who had a fever or other symptoms that could suggest coronavirus should get tested, even if they have been vaccinated.

Meanwhile a Government-backed study is being launched to determine whether different coronavirus vaccines can safely be used for the first and second doses.

The programme, which has received £7 million in funding from the Government’s Vaccine Taskforce, aims to establish whether a mixed-dose vaccine regimen is better than, or a good alternative to, using two doses of the same Covid-19 jab.