The most common adverse effects are pain at the site of injection (> 80%), fatigue (> 60%), headache (> 50%), myalgias (muscle pain) and chills (> 30%), arthralgias (joint pain) (> 20%), fever and inflammation at the injection site (> 10%), mostly mild or moderate in intensity and disappearing within a few days after vaccination. These reactions are more common after the second dose and their frequency decreases with age.
In addition, very rare cases of myocarditis and pericarditis (inflammation of the heart muscle or membrane around the heart) have been observed. These cases have occurred mainly in the 14 days following vaccination, with greater frequency after the second dose of vaccination, and more commonly in younger men. Myocarditis and pericarditis are mild in most cases and resolve with treatment and rest.
Cases of anaphylaxis have been reported. However, anaphylaxis to the mRNA COVID-19 vaccines is currently estimated to occur in 2.5 to 11.1 cases per 1 million doses, largely in individuals with a history of allergy. Anaphylaxis is treatable if recognized early and treated promptly.
If a person had a severe allergic reaction after getting a shot of an mRNA COVID-19 vaccine (either Pfizer-BioNTech or Moderna), that person should not get another shot of that vaccine.
Update (March 2023):
Overall, the safety profile associated with a third dose of BNT162b2 (30 µg) administered approximately 6 months after completing the two-dose regimen is very similar to the safety profile of the initial regimen itself, with no new safety concerns identified in the those who received a booster and with no increased reactogenicity or unusual adverse events or other safety findings.