According to Johns Hopkins medical experts there are a few well documented side effects of the different vaccines. The following side effects are those which might require medical attention although vaccines may cause flu-like symptoms the day after due to an immune response.
The first serious side effect is Myocarditis or inflammation of the heart muscle, which is sometimes accompanied by pericarditis, or inflammation of the lining outside the heart. This is a side effect known to occur after receiving the Moderna or the Pfizer-BioNTech vaccines. This condition is relatively rare, relatively mild, when associated with the vaccine, and generally resolves itself, however any chest pain should be taken seriously in the weeks following the vaccine. The likelihood of this side effect is very low. As of June 11, 2021 there were about 1,226 reported cases of post-vaccination myocarditis out of the about 296 million doses of mRNA COVID-19 vaccine administered. This is most likely to occur in men under 30 about three days after infection. About three quarters, 76%, of the cases occurred after the second dose of a two dose vaccine.
On the other hand, the single shot Johnson & Johnson vaccine, called Janssen, has documented side effects including Guillain-Barré syndrome (GBS), a rare, neurological, paralyzing, autoimmune disorder as well as thrombosis with thrombocytopenia syndrome (TTS). Both of these side effects, although very rare, are potentially serious and anyone considering their options should research the risk factors associated with each vaccine and what vaccination options are available to them. For reference, as of June 30, there have been 100 cases of GBS reported in the U.S. out of 12.2 million doses of the Johnson & Johnson vaccine that have been administered. As of April 12 2021, there were six cases of TTS reported in the U.S. out of 6.8 million doses administered. For reference, the background rate for TTS expected in the general population ranges from one to five cases per 100,000 people depending on risk factors as well as specific type of thrombocytopenia.
Less serious side effects that may occur after any of these vaccinations include fever, headache, fatigue, muscle pain, chills, diarrhea and pain or stiffness at the injection site. These side effects may or may not occur and may vary depending on which vaccine you receive and from person to person. It is important to understand that millions of people both in the U.S. and worldwide have received vaccines and organizations remain vigilant for possible health complications of vaccination.