Mild Systemic Adverse Effects Experienced by 1 in 4 Individuals Given Studied COVID-19 Vaccines

The data found fewer adverse effects in the general population with both the Pfizer-BioNTech and AstraZeneca vaccines than reported in trials.

One in 4 individuals who received the Pfizer-BioNTech or AstraZeneca vaccine against the coronavirus disease 2019 (COVID-19) experience mild, short lived systemic adverse effects, according to a study published in the Lancet Infectious Diseases. Headache, fatigue and tenderness are the most common symptoms experienced, most of which peaked within the first 24 hours following vaccination, and usually lasted 1 to 2 days.

The analysis of data was pooled by investigators with King’s College in London from the ZOE COVID Symptom Study app. The data found fewer adverse effects in the general population with both the Pfizer-BioNTech and AstraZeneca vaccines than reported in trials. The researchers also found a significant decrease of infection rates from 12 to 21 days after the first dose of the Pfizer-BioNTech and AstraZeneca vaccines compared to a control group, with a 58% and 39% reduction respectively. The drop in infection at least 21 days after the first dose for Pfizer is 69% and for AstraZeneca 60%.

According to the study, 25.4% of vaccinated people indicated suffering from one or more systemic adverse effects, whereas 66.2% reported one or more local adverse effects at the injection site. By dosage, 13.5% of participants reported adverse effects after their first Pfizer dose, 22.0% after the second Pfizer dose and 33.7% after the first AstraZeneca dose. Adverse effects were more common among participants under 55 years of age and among women.

Participants who had a confirmed case of prior COVID-19 were 3 times more likely to have systemic adverse effects after receiving doses of the Pfizer vaccine than those without known infection and almost twice as likely after the first dose of the AstraZeneca vaccine. These individuals were also more likely to experience local effects.

Rates of adverse effects were much lower than expected from clinical trials, and rates of infection after vaccination after 2 to 3 weeks were in line with findings from previous trials and recent data from the Israeli vaccination program.

“The data should reassure many people that in the real world, aftereffects of the vaccine are usually mild and short-lived, especially in the over 50's who are most at risk of the infection,” said Tim Spector, OBE, FMedSci, professor of genetic epidemiology at King’s College, in a press release. “Rates of new disease are at a new low in the UK according to the ZOE app, due to a combination of social measures and vaccination and we need to continue this successful strategy to cover the remaining population.”

The AstraZeneca vaccine is not FDA approved, and is not available to patients in the United States. The FDA has granted Emergency Use Authorization to the Pfizer-BioNTech vaccine, as well as COVID-19 vaccines from Moderna, and Johnson & Johnson’s Janssen.


Only one in four people experience mild systemic side effects from COVID-19 vaccines [news release]. EurekAlert; April 27, 2021. Accessed April 28, 2021.