Although there is no preference for either the Pfizer-BioNTech or Moderna vaccine, experts agree that the COVID-19 vaccines are not interchangeable between the first and second doses. The safety and effectiveness of mixed vaccines has not be studied and more research is needed.
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According to the CDC, every effort should be made to ensure patients receive the same vaccine for both the first and second dose. This includes things like providing vaccine record cards, recording the information in the patient’s medical records and logging the information in the immunization information system (IIS).
That being said, the CDC has stated that in truly exceptional situations, in which the first-dose vaccine cannot be determined or is no longer available, either vaccine may be given at a minimum of 28 days to complete the vaccination series.
But in normal circumstances, you should receive the same vaccine product for both doses.
– Primary care specialist, Michelle Medina, MD.