Safety of Simultaneous mRNA COVID-19 Vaccine With Other Childhood Vaccines in Young Children

Inclusion Criteria: * Child 6 months through \<5 years of age at time of enrollment. * Child is due to receive mRNA COVID-19 vaccine and at least one other routinely recommended non-live vaccine per CDC or ACIP recommendations. * Parental/LAR intention of child receiving mRNA COVID-19 vaccine and at least one recommended non-live vaccine. * The parent/LAR must be willing and capable of providing permission for their child to participate through the written informed consent process. * The parent/LAR must be available for follow-up and must at minimum have telephone access. * The parent/LAR must agree to sign a medical release for the child so that study personnel may obtain medical information about the child's health (if needed). * The parent/LAR must be willing to delay COVID-19 vaccination for their child for up to 3 weeks. * The parent/LAR must be able to read English or Spanish. Exclusion Criteria: * History of any seizure (including febrile seizure) or first degree relative (biologic parent or biologic sibling including half-sibling) with a history of febrile seizure. * Contraindication to mRNA COVID-19 vaccine: A history of a severe allergic reaction (e.g., anaphylaxis) after a previous dose or to a component of the COVID-19 vaccine or a known diagnosed allergy to a component of COVID-19 vaccine. * A history of a severe allergic reaction (e.g., anaphylaxis) after a previous dose or to a component of a vaccine administered on the day of study enrollment. * For children receiving DTaP vaccine (alone or combination vaccine): Encephalopathy (e.g., coma, decreased level of consciousness, prolonged seizures), not attributable to another identifiable cause, within 7 days of administration of previous dose of DTP or DTaP. * Received any other non-live vaccines within 14 days prior to enrollment or any other live vaccines within 28 days prior to enrollment. * Intention to receive non-COVID-19 non-live or live vaccines during the 4 weeks after Visit 1; vaccines may be administered after enrollment if deemed a personal or public health priority by the health care provider caring for this patient or the study team. * Received prior COVID-19 vaccine as part of a clinical trial. * Received any experimental/investigational agent (vaccine, drug, biologic, device, blood product, or medication) within 28 days prior to enrollment in this study or expects to receive an experimental/investigational agent during the study. * A moderate to severe acute illness and/or a reported temperature ≥ 100.4°F (≥38.0°C) within 48 hours prior to enrollment or a temperature (measured by temporal artery thermometer) ≥100.4°F (≥38.0°C) at the time of enrollment. (This may result in a temporary delay of vaccination). * Receipt of an antipyretic medication (acetaminophen or ibuprofen) within 72 hours prior to enrollment (this may result in a temporary delay of vaccination) or planned receipt of a prophylactic antipyretic medication on the day of and/or days following vaccination prior to any measured increase in temperature in anticipation of a fever (this exclusion does not apply if the Parent/LAR indicates they might administer antipyretics or analgesics after vaccination to reduce a fever or pain). * Immunosuppression as a result of an underlying illness or treatment, or use of anti-cancer chemotherapy or radiation therapy since birth. * Long term (at least 14 days of prednisone 2 mg/kg/day or equivalent other glucocorticoid) use of any parenteral steroids or high-dose inhaled steroids (\>800 mcg/day of beclomethasone dipropionate or equivalent) within the 6 months prior to enrollment (topical and nasal steroids are allowed). * Has an active case of COVID-19 infection. * History of multisystem inflammatory syndrome (MIS-C). * History of myocarditis or pericarditis. * Has any condition that would, in the opinion of the site investigator, place the participant at an unacceptable risk of injury or render the participant unable to meet the requirements of the protocol. * Any child or grandchild of a study investigator or study team member.