A sore arm is the most common side effect of the Covid-19 vaccines, a new study has found. Other common side effects included headache and fatigue.

Analysis of data from the Zoe Covid Symptom Study app reassuringly found fewer side effects in the general population with both the Pfizer and AstraZeneca vaccines than reported in clinical trials by their manufacturers.

One in four people experience “mild, short lived” side effects after receiving either vaccine, the study found. Most peaked within the first 24 hours following vaccination and usually lasted one to two days.

The study by researchers from King’s College London, published in the Lancet Infectious Diseases journal, is the first large scale study to look at the prevalence of mild side effects in the UK’s vaccination programme.

The data came from 627,383 users of the Symptom Study app who self-reported ‘systemic’ and ‘local’ side effects within eight days of receiving one or two doses of the Pfizer vaccine or one dose of the AstraZeneca vaccine. Local effects refer to side effects at the site of injection, in the arm, including swelling, tenderness, redness, itching, warmth and swollen armpit glands, while systemic effects are those that occur elsewhere in the body, include headache, fatigue, chills and shivers, diarrhoea, fever, joint pain, muscle pain and nausea,

Local side effects were the most common, with 66.2% of respondents reporting one or more side effects at the site of where the jab had been administered – typically a tender arm. Meanwhile, one in four (25.4%) vaccinated people indicated suffering from one or more systemic side effects. The most reported systemic side effect was headache, followed by fatigue. 

Side effects were more likely after the first dose of the AstraZeneca jab – 13.5% of participants reported side effects after their first Pfizer dose, 22% after the second Pfizer dose and 33.7% after the first AstraZeneca dose, analysis found.

Participants who’d previously had Covid-19 were also more likely to have side effects after either vaccine. The research also identified that side effects were more common among people under 55 years of age and among women.   

Professor Tim Spector, lead scientist on the Zoe Covid Symptom Study app and professor of genetic epidemiology at King’s College London, hoped the data would help reassure people that the side effects are mostly mild.

He said: “The data should reassure many people that in the real world, after effects of the vaccine are usually mild and short-lived, especially in the over-50s who are most at risk of the infection.”

In clinical trials of the Pfizer vaccine, the most common side effects were pain at the injection site (71-83%), fatigue (34-47%) and headache (25-42%), however the real-world analysis found less than 30% of users complained of injection site pain and less than 10% of fatigue and headache after the first dose.

Similarly, in trials for the AstraZeneca vaccine, systemic side effects were found in 88% of younger participants (18-55 years) after the first dose but this study found a significantly lower rate of 46.2% after the first dose.

The AstraZeneca vaccine recently came under scrutiny when countries stopped using it over fears it might be causing blood clots in some patients. While a link hasn’t been confirmed, as a safety precaution, those aged between 18-29 may be offered an alternative vaccine (either Pfizer or Moderna) instead.

But Dr Cristina Menni, first author of the Kings study, said: “Our results support the after effects safety of both vaccines with fewer side effects in the general population than reported in the Pfizer and AstraZeneca experimental trials and should help allay safety concerns of people willing to get vaccinated.”

How to deal with vaccine side effects

If you have mild side effects like aches, pains and fever, the NHS suggests taking a painkiller such as paracetamol. It’s quite common to get a mild fever after the jab which resolves itself in roughly 48 hours – if you have a fever for longer, or any other symptoms of the virus, consider getting a Covid-19 test.

People who experience any side effects, whether mild or more severe, can report these on the Yellow Card website to help inform vaccine safety.

As a precaution, people should seek urgent medical assistance if they have the following symptoms after the AstraZeneca vaccine: shortness of breath, chest pain, swelling in your leg, persistent abdominal pain, neurological symptoms including severe and persistent headaches or blurred vision, and tiny blood spots under the skin beyond the site of injection.

If you have other symptoms that become worse or start to cause concern, you should seek medical assistance by calling 111 or your GP practice. In an emergency, phone 999.