All vaccines aim to provoke your immune system to produce a protective response that will prevent or mitigate a future infection from the targeted microbes. In some people, that initial immune response might come with a few uncomfortable side effects.
Let's briefly consider the common side effects of some vaccines often given to adults. The list of side effects of the seasonal flu vaccine, for example, includes soreness, redness, and/or swelling where the shot was given, a low-grade headache, fever, nausea, muscle aches, and fatigue. People inoculated with the hepatitis A vaccine sometimes experience swelling, tenderness, redness, warmth, or a hard lump where the shot was given, along with a low-grade fever, a general ill feeling, nausea, and a headache.
Travelers to the Asia-Pacific region are sometimes immunized (as I have been) against Japanese encephalitis. One in four people who contracts that disease dies. One study reported that 46 percent of those vaccinated experienced no side effects at all. However, 54 percent reported about one or more adverse effects. Local reactions at the injection site affected 41 percent of vaccine recipients, while 13 percent had systemic side effects like headache, fever, dizziness, and generalized rash. (Fun fact: Some Japanese encephalitis vaccines are made using kidney epithelial cells extracted from an African green monkey.)
So which side effects can those of us lucky enough to be inoculated with the new COVID-19 vaccines from Pfizer/BioNTech and Moderna expect? According to the Food and Drug Administration's (FDA) evaluation, the "most common solicited adverse reactions were injection site reactions (84.1%), fatigue (62.9%), headache (55.1%), muscle pain (38.3%), chills (31.9%), joint pain (23.6%), [and] fever (14.2%)." Among the 44,000 participants, three "serious" adverse events were reported: one case of inflamed lymph nodes, a shoulder injury (possibly from the administration of the vaccine), and an episode of heart arrhythmia in a participant with known cardiac conditions.
A preliminary report on the Moderna COVID-19 vaccine in The New England Journal of Medicine observed, "Solicited adverse events that occurred in more than half the participants included fatigue, chills, headache, myalgia, and pain at the injection site." The report notes that the side effects were often more pronounced after the second inoculation.
In summary, people inoculated with the new COVID-19 vaccines from Pfizer/BioNTech and Moderna experience the same sort of mild to moderately uncomfortable side effects found in most other vaccines commonly given to adults.
But what about the reports of two people in the United Kingdom who experienced an anaphylactic allergic reaction—their blood pressure dropped suddenly and they had difficulty breathing—to the Pfizer/BioNTech vaccine? It turns out that both of those individuals have a history of severe allergic reactions. The U.K. government's advice is now that the vaccine should not be given to anyone who has ever had an anaphylactic reaction to a food, medicine, or vaccine.
The upshot is that news stories, Facebook posts, and tweets recounting rare dangerous side effects of the new COVID-19 vaccines should not discourage you from getting immunized when your turn comes.