In recipients of one dose of AstraZeneca ChAdOx-1-S vaccine, the Pfizer-BioNTech BNT162b2 vaccine produced an augmented response.
Several COVID-19 vaccines, such as mRNA vaccines (authorized in the U.S.) and the viral vector-based ChAdOx-1-S vaccine (authorized elsewhere), consist of two doses of the same vaccine (i.e., homologous vaccination). There is considerable interest in whether heterologous vaccination — administration of two different types of COVID-19 vaccine — is immunogenic and safe. Now, the CombiVacS trial conducted in Spain provides a preliminary answer.
Participants were adults who had received one dose of the ChAdOx-1-S vaccine within the past 8–12 weeks. In all, 450 in the intervention group received a booster with the Pfizer vaccine while 226 in the control group did not receive any additional vaccination. In the intervention group, anti–SARS-CoV-2 immune responses rose markedly following the Pfizer vaccine (for example, the geometric mean titer of anti-receptor binding domain antibodies increased >75-fold). Neutralizing antibody and T cell responses also increased following the booster. Reactions were mild or moderate, as with homologous vaccination. No participant had a serious adverse event.
