Expiration date on the vaccine and diluent.After 2 hours, return unmixed vials to the refrigerator. Vials can be held at room temperature for up to 2 hours before mixing. Allow vaccine to come to room temperature. Remove vaccine from the freezer or refrigerator.Perform hand hygiene before vaccine preparation, between patients, when changing gloves (if worn), and any time hands become soiled.* People with a contraindication to Janssen COVID-19 vaccine (including due to a known polysorbate allergy) have a precaution to mRNA COVID-19 vaccination.People who have previously received an mRNA COVID-19 vaccine dose should wait at least 28 days to receive Janssen COVID-19 vaccine. People with a contraindication to mRNA COVID-19 vaccines (including due to a known PEG allergy) have a precaution to Janssen COVID-19 vaccination.Vaccination of these individuals should only be done in an appropriate setting under the supervision of a healthcare provider experienced in the management of severe allergic reactions. Healthcare providers and health departments may also request a consultation from the Clinical Immunization Safety Assessment COVID vax Project. ± Consider consultation with an allergist-immunologist to help determine if the patient can safely receive vaccination. †For the purpose of this guidance, an immediate allergic reaction is defined as any hypersensitivity-related signs or symptoms, such as urticaria, angioedema, respiratory distress (e.g., wheezing, stridor), or anaphylaxis that occur within 4 hours following exposure to a vaccine or medication. People with a contraindication to Janssen COVID-19 vaccine have a precaution to both mRNA vaccines (see footnote).This includes people with a reaction to a vaccine or injectable therapy that contains multiple components, one of which is a vaccine component, but for whom it is unknown which component elicited the immediate allergic reaction.History of an immediate allergic reaction † to any other vaccine or injectable therapy (i.e., intramuscular, intravenous, or subcutaneous vaccines or therapies).Note: Persons who have a contraindication to an mRNA COVID-19 vaccine (Moderna or Pfizer-BioNTech) may be able to receive the Janssen COVID-19 vaccine (see footnote). Immediate allergic reaction † of any severity to a previous dose or known (diagnosed) allergy to a component of the vaccine.Severe allergic reaction (e.g., anaphylaxis) after a previous dose or to a component of an mRNA COVID-19 vaccine (Moderna or Pfizer-BioNTech).