Only One in Four People Suffers Mild Side-Effects From COVID-19 Vaccines

One in four people suffers mild, short-lived systemic side effects after the injection of the AstraZeneca or Pfizer vaccine — with headaches, fatigue, and tenderness constituting the most common ones, according to a new study published in the journal Lancet Infectious Diseases.

Most vaccine-related side-effects reached their peak within the first 24 hours after vaccination — and continued for one to two days.

The first large-scale comparison of the AstraZeneca and Pfizer vaccines

The new study is the first large-scale study to compare and contrast the two vaccines — evaluating the prevalence of mild side effects within the U.K.'s vaccination rollout. Researchers from King's College London analyzed the data from the ZOE COVID Symptom Study app — which found far fewer side effects throughout the general population for both AstraZeneca and Pfizer vaccines reported in the trials, according to the embargoed release.

The study also showed a substantial decrease in infection rates from 12 to 21 days after the initial dose of the AstraZeneca (39% reduction) and Pfizer (58% reduction) vaccines, compared to the trial's control group. This decline in infection continued — at least 21 days following the initial dose of AstraZeneca, it had fallen 60%, with a 69% drop observed in Pfizer vaccine recipients.

This large-scale evaluation compared the differences of reported side effects from both vaccines currently rolling out in the U.K. Systemic effects are side effects not happening at the location of injection — like fatigue, headache, chills and shivering, arthralgia, nausea, myalgia, and diarrhea. By contrast, local side effects are those that happen at the site of injection, on the arm — which in this case included pain at the injection region, swelling, tenderness, itching, redness, warmth, and swelling in the glands within the armpits.

This study is based on data gathered from 627,383 users of the ZOE COVID Symptom Study app — all of whom self-reported both local and systemic side effects within the scope of eight days of receiving one or two jabs of the Pfizer or one dose of the AstraZeneca vaccine from December 8 to March 10.

Headaches were the most commonly reported systemic side-effect

A quarter of people who received the vaccines self-reported one or more systemic side effects, but 66.2% reported one or more local side effects. Additionally, 13.5% of participants self-reported side effects following the initial Pfizer dose, with nearly a quarter reporting the same after the second, and a third of participants reported side effects after receiving the AstraZeneca dose. The most commonly reported systemic side effect of the dose was headaches — with 7.8% experiencing headaches after the first Pfizer dose, and 13.2% following the second one. Nearly a quarter of participants who'd received their first dose of the AstraZeneca vaccine reported the development of a headache.

The next-most reported systemic side effect was fatigue — with 8.4% and 14.4% reporting fatigue after the initial and second doses of the Pfizer vaccine, and another 21.1% self-reporting fatigue following their first dose of AstraZeneca's. The most-reported local side effect was tenderness in the area of the shot — with 57.2 and half of participants experiencing this symptom after the first and second dose of the Pfizer vaccine, and another half of AstraZeneca vaccine recipients reporting fatigue after their first injection.

Recently, the CDC's mask mandates were relaxed to allow vaccinated people to go outside without a mask — in addition to unvaccinated people (with conditions). While the study on systemic side effects was carried out in the U.K., knowing the facts and relative mildness of potential side effects from COVID-19 vaccines, in conjunction with the lifting of mask-wearing guidelines, could serve as a greater impetus for more people to schedule and go through with a vaccine regimen, so we can move closer to putting the coronavirus crisis behind us.