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Anyone with a history of immediate allergic reaction of any severity to any component of mRNA COVID-19 vaccines or to polysorbate should not be vaccinated, the CDC said in

While previously the agency released guidelines for contraindication to vaccination, it's now broadened to include those with a history of immediate allergic reaction to a previous dose of or any component of mRNA COVID-19 vaccines, including polyethylene glycol.

In addition, people developing severe allergic reactions such as anaphylaxis after a first dose of mRNA COVID-19 vaccine should not receive a second dose.

All ingredients in the Pfizer/BioNTech and Moderna vaccines were listed by CDC in an "interim clinical considerations" document posted on the agency's website (). Polysorbate is not a vaccine ingredient but was included because people sensitive to it may also react to polyethylene glycol.

People with allergy histories involving these substances should be considered for allergist/immunologist referral to determine if they can receive the vaccine, the agency said in a call with clinicians.

"This guidance is being updated primarily because starting in a few days, people who got their first dose will be eligible to receive a second dose and we thought providers needed more guidance," said the CDC's Sarah Mbaeyi, MD, on the call. "We are continuously reevaluating our guidelines to make sure they are based on the best available information, so providers had more information available when people started coming in for their second dose of vaccines."

CDC guidance defined an "immediate allergic reaction" as any hypersensitivity-related signs or symptoms such as urticaria, angioedema, respiratory distress such as wheezing, or anaphylaxis that occur within four hours following exposure.

But it is important to distinguish allergic reactions from other symptoms, such as vasovagal symptoms or vaccine side effects. Mbaeyi noted about 90% of people with anaphylaxis present with skin findings, "which can be helpful in distinguishing allergic reactions from other reactions."

She added that most immediate allergic reactions occur within 15-30 minutes of vaccination, whereas vaccine side effects, such as fever, chills or fatigue, occur a median of 1-3 days following vaccination.

Those with immediate allergic reaction to any vaccine or injectable therapy not related to a COVID vaccine component or polysorbate requires a balance of the risks and benefits of vaccination. Mbaeyi said in those cases, vaccination may be deferred and the individual may wish to consult with an allergist-immunologist.

Risks may include risk of exposure to SARS-CoV-2, risk of severe disease or death, an unknown risk of anaphylaxis and the ability of the patient to be vaccinated where appropriate medical care is immediately available for anaphylaxis.

However, as before, history of allergic reactions not related to vaccines, injectable therapies or components of mRNA vaccines -- such as food, pet dander, or latex -- is not a contraindication to vaccination, Mbaeyi said.

Those with a precaution to vaccination or history of anaphylaxis due to any cause should be observed for 30 minutes following vaccination, and all other persons should be observed for 15 minutes afterwards.

Tom Shimabukuro, MD, also of the CDC, said that cases of anaphylaxis following COVID-19 vaccination had received outsized media attention. "These are still very rare events," he said.

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