Preliminary results of the CombiVacS study conducted in Spain


To date, the administration of different COVID-19 vaccines has been homologous in nature in the vast majority of cases, with administration of the same vaccine according to its authorized regimen. Adopting a heterologous vaccination regimen – administering the second dose of vaccine using a vaccine different from the one used for the initial dose – could give vaccination programs greater flexibility and make it possible to deal with shortages in a given vaccine. There has also been speculation that a heterologous regimen could result in an even better immune response, in both antibody levels and cellular immune response. 

The CombiVacS study is a Phase 2, multicenter randomized controlled study being conducted at five university hospitals in Spain. A group of volunteers who had received one dose of the AstraZeneca vaccine received a second dose of the Pfizer-BioNTech vaccine between 8 and 12 weeks after the first dose. Immunogenicity was measured 14 days after the second dose, and preliminary results appear to indicate a marked immune response, as well as an acceptable safety profile. 

One limitation of the study was the absence of a control group with a complete homologous vaccination regimen, using the initial vaccine for the second dose; this was not possible because at that time, use of the AstraZeneca vaccine had been suspended in Spain. 

Source: Borobia AM, Carcas AJ, Pérez-Olmeda M, et al. Immunogenicity and reactogenicity of BNT162b2 booster in ChAdOx1-S-primed participants (CombiVacS): a multicentre, open-label, randomised, controlled, phase 2 trial. Lancet

2021; published online June 25.                                                                                                                                

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