Real-world data suggest the side effects from Pfizer and AstraZeneca vaccines are not as bad as expected

• Side effects from the Pfizer and AstraZeneca vaccines are less common in the real world than in trials.
• Research shows that one in four people who got either of those vaccines reported mild side effects.
• Other vaccines, too, might have lower rates of side effects than clinical trials indicated.

Fewer people reported side effects after their Pfizer or AstraZeneca vaccines in the real world than in clinical trials, according to a recent study.

About one in four people who got either vaccine reported mild systemic side effects after their COVID-19 shots, according to the study, which was published in The Lancet Infectious Diseases journal this week. Systemic reactions, by definition, are those that affect the whole body, so the injection-site pain that most people experience isn't included in the number.

The finding was based on data from the ZOE COVID Symptom Study app. More than 620,000 people in the UK shared their vaccine side effects with the app.

According to that data, far fewer people reported fatigue than in clinical trials: Roughly 14% of app users said they experienced fatigue after their second Pfizer shot, while 63% reported feeling fatigued after either dose during the company's clinical trials. Among AstraZeneca recipients, about 21% documented fatigue via the app, compared with 53% in trials.

"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," Tim Spector, the study's lead author and an epidemiologist at King's College London, said in a statement.

The most common side effects were headaches and fatigue

Headaches were among the most commonly reported Pfizer vaccine side effects in the ZOE app. According to the Lancet study, about 13% of vaccine recipients said they experienced a headache after their second dose - a much lower portion than in the trial, where 55% of people reported headaches.

Just under 23% of AstraZeneca recipients reported headaches via the app, compared with the nearly 53% who experienced headaches in trials.

Fewer people reported pain at the injection site in the real world, too. Most Pfizer trial participants - 84% - experienced some pain at the injection site after their shots. In the real-world data, 57% of people reported pain at the injection site after their first Pfizer dose, and 51% after their second. The findings were similar for the AstraZeneca vaccine.

However, since all the real-world data were self-reported on the ZOE app, it's possible that some people forgot to record their side effects or opted not to log all of them.

Because the Lancet study was done in the UK, it didn't include data on the Moderna or Johnson & Johnson vaccines. Neither of those shots has been authorized there yet.

In J&J's clinical trials, less than 50% of people reported arm pain, and around 38% reported headaches and fatigue. Nearly 92% of people in the Moderna clinical trials, meanwhile, reported pain at the injection site after the second dose, while nearly 69% reported fatigue, and 63% reported a headache.

If the trend seen among Pfizer and AstraZeneca recipients holds true for these other vaccines, though, it's possible that real-world recipients in general will report fewer side effects than in clinical trials.