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A sore arm is the most common side effect following a coronavirus vaccine, research suggests.
The UK has three jabs in its immunisation arsenal against the pandemic – Pfizer-BioNTech, University of Oxford-AstraZeneca, and Moderna, with the latter only recently being rolled out.
The vaccines were approved after rigorous testing demonstrated they were both safe and effective.
To better understand the rate of side effects "in the real world", scientists from King's College London analysed more than 627,000 users of the Zoe COVID Symptom Study app.
The users self-reported any adverse event they experienced within eight days of receiving one or two doses of the Pfizer-BioNTech vaccine, or the first dose of the Oxford-AstraZeneca jab, between 8 December, 2020, and 10 March, 2021. Few people had reportedly received their second Oxford-AstraZeneca dose at the time of the study.
Overall, more than two-thirds (66.2%) of users claimed to endure tenderness at the site of their injection.
Perhaps reassuringly, fewer people developed the side effect in real life than in the clinical trials that led to the vaccines' approval, with up to 83% of participants enduring a sore arm in the trial that led to the Pfizer-BioNTech jab being signed off.
"The data should reassure many people that in the real world, after effects of the vaccine are usually mild and short-lived," said lead author Professor Tim Spector OBE.
The scientists found most of the adverse events peaked within the 24 hours post-vaccine and eased after one to two days.
"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," added co-author Dr Cristina Menni.
Pfizer-BioNTech: the side effects to expect
The Pfizer-BioNTech study that led to the vaccine’s UK approval involved more than 43,500 participants, who received either the jab or a placebo shot.
As well as the coronavirus jab warding off severe disease, the "incidence of serious adverse events was low and was similar in the vaccine and placebo groups".
One of the most common side effects was developing mild-to-moderate pain at the injection site within a week of the vaccine, however, less than 1% endured severe discomfort.
The mild-to-moderate cases also tended to resolve within one to two days.
The King's scientists found more than half of the Zoe app's participants endured tenderness at the site of their infection after the first (57.2%) and second (50.9%) Pfizer-BioNTech doses.
In the jab's approval-stage trial, fatigue and headache affected up to 47% and 42% of the participants, respectively, after either dose.
In the King's study, just 7.8% of the participants reported a headache after the first Pfizer-BioNTech dose, rising to 13.2% following the second jab.
Fatigue affected 8.4% and 14.4% of the Zoe app participants after their first and second doses, respectively.
In Pfizer-BioNTech's approval study, "only 0.2% of vaccine recipients and 0.1% of placebo recipients reported fever [defined as a temperature of 38.9°C (102°F) to 40°C (104°F)] after the first dose, as compared with 0.8% and 0.1%, respectively, after the second dose".
"Two participants each in the vaccine and placebo groups reported temperatures above 40°C," wrote the trial scientists.
"Systemic events including fever and chills were observed within the first one to two days after vaccination and resolved shortly thereafter."
These side effects are similar to the "very safe" flu vaccine, which can cause a slight rise in temperature, muscle aches and a sore arm.
Fever rates were slightly higher in the King's study, at 1.5% and 3.8% after the first and second doses, respectively.
When it comes to more serious adverse events, 64 (0.3%) of the approval trial's participants who received the vaccine and six who had the placebo (less than 0.1%) reported swollen lymph nodes.
In the vaccine group, "four related serious adverse events were reported": shoulder injury related to vaccine administration; right axillary lymphadenopathy – abnormal lymph nodes in the right armpit; paroxysmal ventricular arrhythmia – abnormal heart rhythm; and right leg paresthesia – burning or prickling sensation.
Nevertheless, "few" participants in either group dropped out of the trial due to intolerable or dangerous side effects.
In addition, "no deaths were considered by the investigators to be related to the vaccine or placebo", added the trial scientists.
Regarding the King's study, the team wrote in The Lancet Infectious Diseases: "Some severe side-effects might have been missed if app users experiencing them were unable to use the app to report side-effects.
"However, we saw substantially lower rates of severe and mild side-effects than observed in phase 3 trials, making the missing of severe side-effects an unlikely explanation for the lower prevalence of side-effects seen in our data."
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Oxford-AstraZeneca: the side effects that could arise post-jab
The trial that led to the approval of the Oxford-AstraZeneca vaccine is slightly more complex, in that four studies carried out in Brazil, the UK and South Africa varied in how the jab was administered.
Nevertheless, the scientists behind the trial concluded the jab had a "good safety profile with serious adverse events and adverse events of special interest balanced across the study arms". These arms consisted of the vaccine and control groups, where participants were given a saline or meningitis shot.
"There were 175 events, three of which were considered possibly related to either the experimental or a control vaccine," they wrote.
In the King's study, the team has reported just over one in five (22.8%) of those who had a first dose endured a headache, while 21.1% reported fatigue.
When it comes to more severe reactions, haemolytic anaemia – when red blood cells are destroyed faster than they are made – is known to have affected one participant of the approval-stage trial 10 days after receiving the meningitis jab. The reaction was "considered possibly related to the intervention".
A transverse myelitis case, inflammation of both sides of a section of the spinal cord, was reported 14 days after the coronavirus booster. The independent neurological committee later considered it to be a spontaneous event with no known cause.
Two later transverse myelitis cases were also deemed to be "unlikely" related to the vaccine.
A South African participant also endured a fever higher than 40°C two days after a jab, but "recovered rapidly without an alternative diagnosis and was not admitted to hospital". It is unclear if they had the coronavirus vaccine or a control shot.
Four non-coronavirus deaths occurred across the studies – a road traffic accident, "blunt force trauma", homicide, and fungal pneumonia – all of which were considered unrelated to the vaccine.
The Oxford-AstraZeneca jab has since come under fire over a link to blood clots. This has not been definitively proven, with the benefits of being vaccinated outweighing the risks for most.
Moderna vaccine side effects
The Moderna vaccine was not being widely rolled out at the time of the King's study and was therefore excluded from the analysis.
Its phase three trial of more than 30,000 participants "exhibited a favourable tolerability and safety profile".
Among the coronavirus and placebo groups, jabs were generally found to be "well tolerated", with most side effects being "mild" or "moderate".
The most common adverse event after both doses was injection-site pain, affecting 86% of the participants. Headache, fatigue and muscle pain were also reported by some.
"The majority of local solicited ARs [adverse reactions] occurred within the first one to two days after injection and generally persisted for a median of one to two days," wrote the trial scientists.
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When to seek medical attention
Coronavirus vaccine side effects are mild and fleeting in the vast majority of cases.
While fever can occur post-jab, this may also be a symptom of the coronavirus itself. Anyone who develops a raised temperature, new and continuous cough, or a loss of taste or smell should get tested.
According to the NHS, injection-site tenderness, fatigue, headaches, aching muscles, nausea and vomiting are relatively common after a coronavirus vaccine.
If these symptoms worsen or you are concerned, call the non-emergency NHS number 111.
People should not have a coronavirus vaccine if they have ever had a serious allergic reaction to any of the jab's ingredients or against a previous dose of the two-vaccine schedule.
If a severe allergic reaction does occur, it tends to arise within minutes, with trained staff on hand at vaccination sites.
When it comes to blood clots, call 111 immediately if you endure any of the following from around four days to four weeks post-jab:
Severe headache that is worsening or not relieved with painkillers
Headache that is worse when lying down or bending over
Headache that is "unusual for you" and occurs alongside blurred vision, nausea, vomiting, speech problems, weakness, drowsiness or seizures
Rash that resembles small bruises or bleeding beneath the skin
Shortness of breath
Persistent abdominal pain