This evidence synthesis has been compiled by members of the CITF Secretariat and does not necessarily represent the views of all CITF members.

By Marija Djekic-Ivankovic

Mixing COVID-19 vaccine products has been done to maximize the number of people that can be fully vaccinated in settings where vaccine availability is limited or unpredictable. Information regarding immunogenicity and safety aspects of this method is growing: two recent European studies concluded that supplementing the Oxford–AstraZeneca first dose with an mRNA vaccine as the second dose (either Pfizer–BioNTech, or Moderna) triggers an immune response that is stronger than two doses of the Oxford–AstraZeneca vaccine.

Key points – study from Germany:

  • A comparison between the Pfizer–BioNTech and AstraZeneca vaccine as second dose was conducted among 87 healthy and relatively young (mean age of 38 years) individuals who had received AstraZeneca as their first dose.
  • Administering Pfizer’s COVID-19 vaccine as the second dose 4 or more weeks after an initial AstraZeneca dose produced a significantly stronger, and a qualitatively better immune response than when a second dose of AstraZeneca was administered. This was observed particularly when samples were challenged with the Alpha, Beta and Gamma variants of concern.


Key points – study from Sweden:

  • A comparison between the Moderna and the AstraZeneca vaccine as second dose was conducted among 88 healthy people who had originally received AstraZeneca as their first dose.
  • Receiving a second dose of the Moderna vaccine also resulted in a better antibody response compared to a second dose of the AstraZeneca vaccine, especially against the Beta variant of concern.
  • Although adverse reactions were reported more frequently with Moderna compared to AstraZeneca, there were no significant differences in the intensity level of adverse effects.

Read more


Barros-Martins J, Hammerschmidt SI, Cossmann A, Odak I, Stankov M, Ramos GM, Dopfer-Jablonka A, Heidemann A, Ritter C, Friedrichsen M, Schultze-Florey C, Ravens I, Willenzon S, Bubke A, Ristenpart J, Janssen A, Ssebyatika G, Bernhardt G, Münch J, Hoffmann M, Pöhlmann S, Krey T, Bošnjak B, Förster R, and Behrens G. Immune responses against SARS-CoV-2 variants after heterologous and homologous ChAdOx1 nCoV-19/BNT162b2 vaccination. Nat Med 2021.

Normark J, Vikström L, Gwon Y-D, Persson I-D, Edin A, Björsell T. and Dernstedt A. Heterologous ChAdOx1 nCoV-19 and mRNA-1273 vaccination. N Engl J Med. 2021 Jul 14.  DOI:10.1056/NEJMc2110716