Mixing and Matching Vaccines

This page is kind of out of date.

I wrote a big long post laying out the advantages and disadvantages of taking AZ or mRNA after getting a first dose of AZ. That advice was BC-focused, and included considerations about vaccine availability.

This page includes some material from that posting, but focuses on scientific findings about the effectiveness of mixing vs. matching, as opposed to trying to help you make a decision between AZ or mRNA.


Side effects

  • The Com-CoV study found AZ+Pfizer side effects were still mild, meaning “no lasting damage and over pretty quickly”, but possibly quite unpleasant.
  • The tiny Ulm study said that the side effects were “in line with previous reports” and that the Pfizer second shot side effects weren’t as bad as the AZ first shot. (I believe that with AZ+AZ, the first dose’s side effects are worse than the second.) They did not compare AZ+AZ directly to AZ+Pfizer.
  • The small Berlin study said basically the same thing as the tiny German study.
  • The Saarland study found that the side effects of a Pfizer dose after an AZ dose were about the same as Pfizer after Pfizer.
  • This study from Saarland University Medical Center with 216 vaccinees found that side effects were worse for AZ than mRNA on the first dose, but AZ+mRNA was fine and very similar to mRNA+mRNA.

Effectiveness in humans

Studyagainst1st Dose2nd DoseEffectiveness
Swedensymptomatic infectionAZPfizer67%
Swedensymptomatic infectionAZModerna79%
Swedensymptomatic infectionAZAZ50%
Swedensymptomatic infectionPfizerPfizer78%
Swedensymptomatic infectionModernaModerna87%
BCDelta infectionPfizerPfizer92%
BCDelta infectionModernaModerna93%
BCDelta infectionAZAZ70%
BCDelta infectionAZmRNA91%
BCDelta hospitalizationPfizerPfizer98%
BCDelta hospitalizationModernaModerna98%
BCDelta hospitalizationAZAZ93%
BCDelta hospitalizationAZmRNA99%

This study from France didn’t give vaccine effectiveness numbers, but reported that people with Pfizer+Pfizer were twice as likely to get infected as those with AZ+Pfizer. That’s a lot!

Note, however, that the first Pfizer dose was only four weeks before the second dose, while the first AZ dose was twelve weeks before the second dose.


This study from the US looked at antibody levels after a booster (e.g. a third shot after two doses of Spikevax), mixing and matching the brands of the boosters. The gamma globulin serum binding antibody levels (in Binding Antibody Units/mL) 15 days after the booster, in International Units per microlitre were:

J&J primarySpikevax primaryComirnaty primary
J&J booster326.03029.41904.7
Spikevax booster3203.16799.86155.0
Comirnaty booster2549.55195.63409.1

The study authors said basically that mixing was good, but looking at this table, my conclusion is that Spikevax (Moderna) is good. Note, however, that this study was done with a full dose of Spikevax, not the half-dose booster that Moderna is requesting authorization for.

Other tables — for the neutralizing antibody titres, for measurements at different times after the booster — showed similar results.

Note that the authors disclosed some pretty significant limitations on the study: it was small — they say too small to compare between groups — and not randomized. So don’t bet the farm based on this study.