Preliminary results on the benefits of combining different COVID-19 vaccines


Researchers involved in different clinical studies have indicated that combining COVID-19 vaccines from different platforms could trigger a more robust immune response, compared to administering two doses of the same vaccine; this would also simplify immunization efforts in countries facing fluctuating supplies of the various vaccines. 

The Spanish clinical trial known as CombivacS, led by the Carlos III Health Institute, began in Madrid in April 2021, with 663 participants under the age of 60 who had received a first dose of the

Oxford/AstraZeneca vaccine. Two-thirds of the participants were selected at random to receive the Pfizer-BioNTech mRNA vaccine at least eight weeks after receiving their first dose, while the remaining

232 participants were assigned to the control group, which has not yet received a second, booster dose. 

Preliminary results from the trial indicate that those who had received the second dose developed much higher antibody levels than the control group, with laboratory tests showing the presence of neutralizing antibodies against SARS-CoV-2. The increase in antibody titers appears to be greater than in most people who receive two doses of the Oxford/AstraZeneca vaccine, according to data from previous trials. In terms of reactogenicity, the vaccinated group maintained a profile similar to that of the Comirnaty vaccine in homologous vaccination regimens. Participants in the CombivacS trial control group experienced no change in antibody levels.

A similar study conducted in the UK, known as Com-COV, which analyzed a combination of the same two vaccines, found that in people who received a different vaccine for the second dose, there was an increase in symptoms such as fever, between 24 and 48 hours after vaccination, compared to people who received two doses of the same vaccine. The primary immunological result of this Com-VOC trial is expected to be available in June 2021. 


Mix-and-match COVID vaccines trigger potent immune response. Nature 593, 491 (2021) doi:

Heterologous prime-boost COVID-19 vaccination: initial reactogenicity data. The Lancet. May 12, 2021 doi:


  • Share: