Although there has been a great deal of interest in mixing and matching different COVID-19 vaccines, very few published studies have addressed this issue.

On Sept. 9, 2021, Swedish researchers published a small study examining participants who received the AstraZeneca vector virus vaccine, which uses a simian adenovirus (ChAdOx1), that contains the gene coding for the SARS-CoV-2 spike protein. Participants were then boosted with either the same vaccine (ChAdOx1 adenovirus vaccine) or an mRNA vaccine (mRNA-1273), produced by Moderna (Normark J, Vikström L, Gwon Y-D, et al. Heterologous ChAdOx1 nCoV-19 and mRNA-1273 Vaccination. N Engl J Med. 2021 Sep 9;385(11):1049-1051).

Among participants, 37 chose the homologous ChAdOx1 booster and 51 chose the heterologous mRNA-1273 booster. Researchers found that after the homologous boost, levels of spike protein-specific IgG were five times higher than on the day of the boost. On the other hand, those boosted with mRNA-1273 had levels of spike protein-specific binding antibodies that were 115 times higher. Similarly, virus-specific neutralizing antibodies were 20 times higher after the heterologous boost but only two times higher after the homologous boost. In addition, neutralizing immune responses against the delta variant were greater after the heterologous boost.

As we look ahead to booster dosing, these data might be relevant for those who previously received one dose of the Johnson & Johnson/Janssen vaccine, which, like the AstraZeneca vaccine, is a vectored adenovirus vaccine.

Materials in this section are updated as new information and vaccines become available. The Vaccine Education Center staff regularly reviews materials for accuracy.

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