Like all new drugs, the vaccines that have been authorized in Western countries to protect against Covid-19 come with some safety concerns and side effects. Many people who’ve received the first two shots deployed, one from Pfizer Inc. and BioNTech SE and another from Moderna Inc., have experienced fever, headache and pain at the site of the injection. These side effects generally disappear quickly. At least a couple of dozen have had a serious allergic reaction, called anaphylaxis, to the vaccines, which were about 95% effective in preventing Covid in clinical trials. No fatalities have been reported, and health care workers are monitoring vaccine recipients to treat the reactions that arise.
The body fights foreign invaders through a variety of mechanisms that include making protective proteins called antibodies, releasing toxins that kill microbes, and marshaling guardian cells to battle the infection. As in any conflict, sometimes the effort to repel an infection can itself be damaging. In rare cases, it can produce runaway inflammation and swelling of tissues in a serious allergic reaction called anaphylaxis. As much as 5% of people in the U.S. have had such a reaction to various substances. It can be fatal if, for example, the person’s airway swells shut, though deaths are rare. Allergies to insect stings and foods can provoke it, though drug reactions are the most common cause of anaphylaxis fatalities in the U.S. and U.K.