Mixing AstraZeneca and mRNA Vaccines

1/ Protection after two doses

The most important decision is to get second dose: stronger + longer lasting protection. Two doses of AZ effective at preventing serious illness from OG Covid-19 + early variants. Early studies show good immune response when Moderna/Pfizer used as second dose.

2/ Protection against new variants

Delta variant spreading quickly. Two doses protect against Delta much better than one. Two doses of Moderna/Pfizer appear to offer more protection against symptomatic infection from Delta than two doses of AstraZeneca.

3/ Risk of blood clots

AZ is linked with a rare but serious blood clotting dx called VITT. Risk: 17 cases of VITT for every 1M people given dose 1 and 2 for every 1M given dose 2. Estimates may change as more people get dose 2. Moderna/Pfizer vaccines do not cause VITT.

4/ Side effects

Common to have side effects like sore arm, fatigue, headache, fever, or sore muscles after any vaccine. Com-COV study in UK showed AZ followed by Pfizer 4 weeks later had more side effects than those who had 2 doses of AZ. Side effects mild and lasted 1-2 days.

5/ Efficacy

  • German study (Barros-Martins et al) studied AZ for dose 1 and then AZ or Pfizer for dose 2. Early results show that the AZ-Pfizer combination had a strong immune response against many of the variants compared to 2 doses of AZ. Additional data outlined below and within Decision Guide (see link below).
  • Study from UK (Bernal et al) suggests 1 dose of AZ 33% effective against symptomatic infection from Delta. 2 doses 60% effective. Same study showed 1 dose of Pfizer is 33% effective and 2 doses 88% effective against symptomatic infection from Delta variant.

6/ Mixing and matching

Occurs for many other vaccines like Influenza, Hepatitis A. Other countries also using mix-and-match strategy for Covid-19 vaccines. Norway those who received AZ for dose 1 will be given mRNA for dose 2. Germany and France also allowing.

7/ Re-starting series

NACI guidance says you are “fully immunized” once you have dose 1 of AZ and dose 2 of AstraZeneca, Moderna or Pfizer. Do not need to re-start. This approach being used in other countries as well.

8/ Other considerations:

  • How did you feel after first dose: Anxious or worried?
  • Availability: is there a steady supply of stock?
  • Access: how long will you have to wait for your choice?

See Decision Guide for additional details. This was developed by a group of doctors, pharmacists, scientists, and patients in Canada to make it easier to understand the evidence and expert guidance (and includes me as an author).

Current immunogenicity data on heterologous prime-boost regimens summarized in a table below: