Let’s begin with the evidence we have so far around mixing and matching vaccines, specifically AstraZeneca and Pfizer/BioNtech (Pfizer). On May 12, initial data on reactogenicity (the ability to produce common side-effects) data from the COM-CoV study in the United Kingdom was released. It included 830 people ages 50 and older, who were randomized into four study arms that received different combinations of AstraZeneca and Pfizer vaccines at four-week dosing intervals.
Participants who received different vaccines for their first and second doses, regardless of sequence of vaccination, had more side-effects (non-serious ones that resolved on their own) than those who received the same vaccine twice. No safety concerns were noted.
Experts theorized that the greater number of side-effects might predict a more robust immune response, but immunogenicity (the ability of the vaccine to provoke an antibody response) data is still pending and expected later this month.
Results from the Spanish CombiVacS study were reported on May 18. The study randomized 663 people who received AstraZeneca as their first dose to either receive Pfizer as a second dose booster eight weeks later, or into a control group with no second dose at all.
Those who received AstraZeneca followed by Pfizer developed twice as many antibodies as historically seen in people who received two doses of AstraZeneca alone. No safety concerns were identified.
A recent study from Germany released June 1 as a non peer-reviewed preprint adds additional information around mixing and matching AstraZeneca and Pfizer vaccines. This preliminary data included 26 individuals, ages 25 to 46, who were administered AstraZeneca as their first dose of vaccine, followed by a second dose of Pfizer given eight weeks later.
Neutralizing activity was 3.9 times greater against the alpha (B.1.1.7) variant and similar against the delta (B.1.617.2) variant compared to neutralizing activity seen in people who received two doses of Pfizer vaccine. No safety concerns were noted.