According to the CDC, either of the currently authorized mRNA COVID-19 vaccines can be used when indicated; ACIP does not state a product preference. However, these mRNA COVID-19 vaccines are not interchangeable with each other or with other COVID-19 vaccine products. The safety and efficacy of a mixed-product series have not been evaluated. Both doses of the series should be completed with the same product. Every effort should be made to determine which vaccine product was received as the first dose, in order to ensure completion of the vaccine series with the same product.
In a recent January 21st update to its Interim Clinical Considerations, the CDC has recommended that in exceptional situations in which the first-dose vaccine product cannot be determined or is no longer available, any available mRNA COVID-19 vaccine may be administered at a minimum interval of 28 days between doses to complete the mRNA COVID-19 vaccination series. If two doses of different mRNA COVID-19 vaccine products are administered in these situations (or inadvertently), no additional doses of either product are recommended at this time.
It is worthwhile to note that Britain has also allowed mixing of products on rare occasions where the initial brand is not available or if the initial brand is not known.
This issue of mixing vaccine products is now being studied. The University of Oxford is conducting a trial evaluating the feasibility of using a different vaccine for the initial ‘prime’ vaccination to the follow-up ‘booster’ vaccination.
Updated: February 9, 2021