Mixing vaccines (AZ and Pfizer) is safe.

A few days ago, the Lancet published a correspondence about the highly anticipated results from an ongoing study in the UK. This study is testing the safety and effectiveness of heterologous prime boosting (i.e. mixing vaccines).

This was published as a correspondence instead of a full peer reviewed study because the authors are still waiting on efficacy results. The scientists had the safety data though and were given the “okay” by the data safety board to publish. MANY people around the world were waiting for this, as supplies are short, people are concerned about AstraZeneca (AZ) blood clots, many are concerned about B.1.351 (variant first discovered in S. Africa), etc.

The study

  • Recruitment began on Feb 11, 2021 and completed on Feb 26, 2021

  • 463 participants were randomly assigned to one of four groups:

    1. AstraZeneca at baseline and Pfizer 28 days later (AZ+BNT)

    2. Pfizer at baseline and AstraZeneca 28 days later (BNT+AZ)

    3. Pfizer at baseline and Pfizer 28 days later (BNT+BNT)

    4. AstraZeneca at baseline and AstraZeneca 28 days later (AZ+AZ)

  • The average age of participants was 57 years (range 50-69), 46% were female, and 25% from ethnic minorities


  • Mixing doses induced greater symptoms than not mixing

    • Fever in 34% AZ+BNT and 41% BNT+AZ compared to 10% for AZ+AZ and 21% for BNT+BNT

    • Similar increases were observed for chills, fatigue, headache, joint pain, discomfort, and muscle aches (see Figure)

  • No hospitalizations due to side effects

  • Majority of side effects happened within 48 hours of the vaccine

  • No thrombocytopenia (blood clots) in any group at day 7 (although this was expected, they needed a whole lot more people in the study to “truly” see an effect if a safety signal was present)

Bottom Line: Overall, it looks safe to mix AstraZeneca with Pfizer. There is a disadvantage to mixing (i.e. higher rate of side effects), but this is short lived. We’re anxiously waiting the efficacy results, which are expected in June 2021.

The authors also hinted that they’re incorporating vaccines manufactured by Moderna and Novavax and expect results soon.

Love, YLE