Common side effects of the COVID-19 vaccines include injection site pain, fatigue, headache, muscle pain and fever.
Data from the Pfizer/BioNTech and Moderna trials also show the side effects included joint pain and chills. These reactions are more likely after the second dose of those vaccines, given several weeks after the first, and are more common and severe in younger people. The Johnson & Johnson side effects, too, were more common in 18- to 59-year-old participants, and also included nausea.
As the CDC explains, these common side effects are normal signs that your body is beginning to mount an immune response and the symptoms should go away within several days. Some people have no side effects, and that is normal, too.
Rarely, the COVID-19 vaccines may cause more serious problems. This includes a severe allergic reaction known as anaphylaxis, which has occurred in 2 to 5 people per million vaccinated, according to the CDC, and, for the Johnson & Johnson vaccine, a blood clotting disorder coupled with low levels of platelets (thrombosis with thrombocytopenia syndrome). As of July 12, that issue, which has almost entirely affected women below the age of 50, has been confirmed in 38 cases, after more than 12.8 million doses of the J&J vaccine.
The FDA also has warned of an observed increased risk of the neurological disorder Guillain-Barré Syndrome associated with the J&J vaccine. The warning came after preliminary reports of 100 cases including one reported death, but it isn’t known whether the death or the cases were caused by the vaccine.
Additionally, there is emerging evidence that the Pfizer/BioNTech and Moderna mRNA vaccines may very rarely cause inflammation of the heart muscle (myocarditis) or of the surrounding lining (pericarditis), particularly in young men.
For more information on these rare adverse events, see “How safe are the vaccines?”