One in four people vaccinated with Pfizer or AstraZeneca experiences mild side effects such as headache, weakness and soreness: symptoms usually culminate in the first 24 hours after administration and disappear within a day or two.
This is demonstrated by the first comparative study of the two vaccines conducted in the general population on almost 630,000 British citizens, therefore outside the controlled conditions typical of trials.
The results, better than those obtained during phase three clinical trials, are published in the Lancet Infectious Diseases journal by researchers from King's College London. The study is based on reports made spontaneously by citizens via an app in the eight days following vaccination, between December 2020 and March 2021.
The sample of 627,000 Britons includes 282,000 people who received the first dose of Pfizer and 28,000 who also received the second, as well as 345,000 citizens who were injected with the first dose of AstraZeneca. Systemic side effects (such as headache, weakness, chills, nausea, diarrhea, fever, joint and muscle pain) were reported by 13.5% of patients after the first dose of Pfizer, up from 22% after the second Pfizer. , and by 33.7% of the people who had made the AstraZeneca premiere.
The most affected were the under-55s and women. For those who had previously had Covid-19, the likelihood of experiencing systemic side effects was three times greater after Pfizer and two times greater after AstraZeneca. As regards the efficacy of vaccines, after 12 days from the first dose there was a significant reduction in the risk of infection, which between 21 and 44 days reached 60% with AstraZeneca and 69% with Pfizer, while between 45 and 59 days has reached 72% with Pfizer.