On the eve of the US FDA meetings some long awaited results from the US Mix and Match booster trial.


➡️ A boost of ANY vaccine raised antibodies a lot, no matter your first type of vaccine.

➡️ Boosting with an mRNA vaccine (Moderna or Pfizer) after an original Johnson & Johnson vaccine produced a higher immune response than boosting with J&J.

➡️ Boosting with J&J AFTER an mRNA prime did not boost quite as well as a third mRNA dose

➡️ For those whose original series was Pfizer or Moderna, boosting with either mRNA vaccine provided a similar immune boost.

❓What did the trial test?

This was a Phase I/II trial, meaning it tested for safety, reactogenicity (side effects), and immune response (antibodies via blood tests) in a relatively small group of participants. It was not large enough to test whether different vaccine combinations led to different chance of actually getting COVID-19 as is done in Phase III trials.

453 adults were enrolled who had completed a primary series of either Johnson & Johnson (one dose), Moderna (two doses) or Pfizer (two doses) at least twelve weeks prior and had no prior history of SARS-CoV-2 infection.

💥 All possible combinations of booster doses were trialled, for a total of nine different groups.

Immune response (binding and neutralizing antibodies) was assessed just prior to booster vaccination and at Days 15 and 29 after the boost.

The attached colorful figure summarizes the results for immune response for each combination.


One wrinkle is that Moderna has asked for boosters at half their original dose, but a full dose booster was tested in this trial.

These data also show immune response in the blood, not exactly how this corresponds to protection in real life.


Booster recommendations likely coming soon for J&J and Moderna in the US.

These results will be discussed this week by the FDA, which we will be sure to report back on.

Based on these data (which are consistent with previous data boosting Astrazeneca with an mRNA dose ), it also seems *likely* that mRNA boosters would be allowed or even recommended for one dose Johnson and Johnson recipients….but we will have to wait and see.

Those Nerdy Girls

*Cheat sheet for figure*:

Top row= binding antibodies (bind to the virus and alert the immune system)

Bottom row= neutralizing antibodies (blocks the attachment and entry of the virus into the cell).
mRNA-1273= Moderna

Ad26.COV2.S= Johnson & Johnson

BNT162b2= Pfizer

Day 29 results not yet available for all groups


NPR – A study of COVID vaccine boosters suggests Moderna or Pfizer works best

Mix and Match trial results (pre-print)

Link to Original FB Post