More than a third of people who get their first dose of the Covid vaccine report side effects, according to a new study.
Most of the side effects were mild and included pain or swelling at the injection site.
The latest data from the Zoe Covid Symptom Study App states that the most common mild (systemic) whole body side effects were fatigue (9%), headache (8%), and chills or chills (4%).
The vaccines use a harmless version or component of the virus to train the immune system so that the body can fight off the virus if it is encountered.
This reaction can feel like some of the symptoms when the body is fighting off a real infection.
This may include headache, fever, chills or chills, tiredness (tiredness), muscle or joint pain, diarrhea, and feeling sick (nausea).
Experts say that a stronger response may indicate an increased immune response.
The data suggests that people who previously had Covid-19 were almost twice as likely to have one or more mild (systemic) side effects all over their bodies than those who did not have the virus (33% vs 19%) from a Pfizer / BioNTech vaccine dose.
In addition, most of the mild (systemic) total body side effects occurred in the first two days after vaccination, and only 3% of people experience after-effects after three days.
The numbers are based on a subsample of nearly 40,000 vaccine doses.
They suggest that 37% of people had local side effects after the first dose and 45% after the second.
Fourteen percent of people reported at least one full body side effect within seven days of the first dose, compared with 22 percent after the second dose.
This may indicate a stronger immune response after the second dose.
The data found that 13% of the vaccinated men and 19% of the vaccinated women reported at least one systemic side effect within seven days.
Those under 55 were more likely to experience full body side effects than those over 55 (21% versus 14%).
These current data are for the Pfizer / BioNTech vaccine.
Most of the people analyzed were healthcare workers.
Tim Spector, lead researcher on the study and professor of genetic epidemiology at King’s College London, said, “This dataset is a unique look at those who have been vaccinated in the real world outside of studies and at post-vaccination effects to date are mild and in the minority of people.
“It is interesting to see that these mild after-effects occur more often in people with previous Covid than in naive subjects.
“This could be good news as a bigger response like this suggests those who get a first dose after Covid will have a stronger immune response and possibly better protection from just a single vaccination.”