Jan. 6 (UPI) -- A severe allergic reaction to the COVID-19 vaccine is "exceedingly rare," reported by 21 of 1.9 million people in the United States who have received the vaccine, the Centers for Disease Control and Prevention announced Wednesday.
Of those 21 who experienced anaphylaxis after being vaccinated, 17 had a history of similar reactions to other vaccines or medications, the CDC said. All 21 recovered, though some required hospital care.
The reaction is "exceedingly rare" and should not discourage most people from getting vaccinated against COVID-19, Dr. Nancy Messonnier, director of CDC's National Center for Immunization and Respiratory Diseases, said on a conference call with reporters.
Based on the data from the initial rollout of the vaccine, the CDC recommends people get the shot unless they have a history of allergic reactions to component ingredients.
"We all hope a vaccine would have zero adverse events," Messonnier said. "But we're in the setting of 2,000 COVID-19 deaths per day, [so] the risk from COVID-19 and poor outcomes from COVID-19 are still higher" than the risk for anaphylaxis following vaccination.
The data covers vaccine doses administered through Dec. 23.
The agency has since documented eight additional cases of anaphylaxis in 4.8 million people who received the vaccine between mid-December and Wednesday.
The U.S. Food and Drug Administration has approved two COVID-19 vaccines, manufactured by Pfizer-BioNTech and Moderna. Both use similar medical technologies and require two doses for maximum effectiveness.
Cases of anaphylaxis -- a severe, potentially life-threatening reaction that can occur within seconds or minutes of exposure to an allergen -- occurred with both vaccines and "multiple lots" or shipments, according to the CDC.
The cause of the reaction in response to the vaccines remains unknown.
Symptoms of anaphylaxis include a skin rash, nausea, vomiting, difficulty breathing and shock. In most cases, it can be treated quickly with the drug epinephrine, or the "epi pen."
In the cases following COVID-19 vaccinations, the average time of onset of anaphylaxis symptoms was 13 minutes.
The CDC is not discouraging vaccination for people with a history of anaphylaxis, but advises them to "be observed for 30 minutes" after receiving the shot, Messonnier said.
All COVID-19 vaccine recipients should be observed for at least 15 minutes after getting the shot.
Vaccine facilities are also urged to be prepared to treat patients for anaphylaxis and have epi pens available in case they are needed.
People who experience an immediate reaction such as anaphylaxis after the first dose of the vaccine should not get the second, the CDC said.
The rate of reactions amounts to 11.1 cases of anaphylaxis per 1 million doses administered, Messonnier said.
In comparison, the typical season flu vaccine sees 1.3 cases of anaphylaxis per 1 million recipients, she said.
"The rate of anaphylaxis with the COVID vaccines may seem high compared to flu vaccine, but this is still a rare outcome," Messonnier said. "It's paramount people get vaccinated," she said.