Study finds that 25% of vaccinated people suffer from post-immunization effects, ranging from mild to life-threatening
05/13/2021 // Ramon Tomey // Views

A British study using real-world data has shown that 25 percent of vaccinated people experience mild side effects after COVID-19 immunization. Researchers looked at data they obtained through an app that tracks the COVID-19 pandemic in the U.K. for their paper. They found that the mild side effects – the most common being headache – usually lasted two days before disappearing on their own.

King's College London scientists led by epidemiology professor Tim Spector analyzed data from the ZOE COVID Symptom Study App. They looked at information provided by more than 620,000 users who reported side effects. The users received either the Pfizer/BioNTech or AstraZeneca COVID-19 vaccine within the period of Dec. 8 and March 10. Both vaccines are approved for use in the U.K., alongside the Moderna mRNA vaccine candidate.

The researchers' study published April 27 in Lancet Infectious Diseases said that 25.4 percent of the vaccinated users reported mild systemic side effects. These side effects not located in the injection site included headache, fatigue, chills, diarrhea, joint and muscle pain and nausea. The users also reported local side effects – limited to the injection site – such as arm pain, redness, tenderness and swelling.

Based on their findings, 13.5 percent of people who got their first dose of the Pfizer/BioNTech vaccine reported side effects. Twenty-two percent of people who got the second dose of the Pfizer/BioNTech vaccine said they experienced these mild conditions. However, 33 percent of people who got the AstraZeneca COVID-19 vaccine experienced mild side effects.


The users reported headaches as the most common side effect post-vaccination. The side effect was experienced by 22.8 percent of AstraZeneca vaccine recipients. Some 7.8 percent of first-dose Pfizer vaccine recipients reported the condition, while 13.2 percent of second-dose Pfizer vaccine recipients said they suffered from these headaches.

The researchers noted in their study that headache, fatigue and fever were more prevalent in those immunized with the British pharmaceutical firm's vaccine. It is worth noting that the data did not include figures for blood clots in the brain reportedly linked to the shot.

Vaccinated people are still at risk from more serious side effects, however

Clinical trials for the Pfizer and AstraZeneca vaccines in 2020 showed more instances of mild side effects vis-à-vis the April 2021 study. Data from Pfizer's phase three trial showed 77.4 percent of volunteers experiencing at least one systemic side effect after the first dose. AstraZeneca trials showed a slightly lower percentage, with 75.8 percent of volunteers experiencing these effects after the first dose.

Spector remarked that the study's results served to placate fears of people regarding vaccines. "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," he remarked.

Study co-author Dr. Cristina Menni added: "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. [This] should help allay safety concerns of people willing to get vaccinated."

Both vaccines require two full doses to work, although with significant differences. The Pfizer vaccine made in partnership with German company BioNTech uses an mRNA vector. On the other hand, the AstraZeneca vaccine made in partnership with the University of Oxford uses an adenovirus vector. Regardless of the manufacturer and mechanism, both vaccines have been linked to serious adverse reactions that have led to fatalities.

Back in March, two health workers in Denmark suffered from blood clots and brain hemorrhage after getting the AstraZeneca vaccine. The two developed the conditions less than two weeks after they were immunized. Because of these cases, the Scandinavian country temporarily suspended use of the vaccine on March 11. Danish health authorities eventually banned the vaccine from the country's COVID-19 immunization program. (Related: Danish regulator says woman who died after AstraZeneca jab had "unusual symptoms" including from blood clots.)

Meanwhile, a medical professional in the U.S. state of Florida died 16 days after his first dose of the Pfizer COVID-19 vaccine. Fifty-six-year-old Dr. Gregory Michael got vaccinated on Dec. 18, and sought medical treatment after seeing red spots on his extremities. Michael's fellow doctors said he developed a condition called acute immune thrombocytopenia, which negatively impacted his blood's clotting ability. He eventually died in early January 2021, with a subsequent autopsy concluding that he died of "natural causes." (Related: mRNA vaccines for the Wuhan coronavirus caused blood disorder in at least 36 people.)

Visit to read more articles about the side effects of getting COVID-19 vaccines.

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