UW Virology Research Clinic

January 5, 2022

NIAID Letter to Mix & Match Trial Participants

December 2021

Thank you for participating in the National Institute of Allergy and Infectious Diseases’ (NIAID’s) Mix and Match COVID-19 vaccine study. Your participation has made a significant public health impact and has resulted in the approval and recommendation of mix and match booster vaccines for adults in the United States. The study team would like to share some of what we’ve learned so far.

We now have early results from the participants who received their original COVID-19 vaccine regimen before they joined the study, then got a COVID-19 booster dose during the trial of either the Moderna vaccine (standard 100 mcg dose), the Janssen vaccine, or the Pfizer-BioNTech vaccine.

First, the boosts appear to be safe. Participants had some mild side effects that are typical of COVID-19 vaccines, but there were no safety concerns.

Second, all three vaccine boosters elicited a good immune response regardless of which original COVID-19 vaccine regimen the participant had received. There were increases in levels of both antibodies (proteins the body produces to fight infections) and specialized cells (T cells) that fight infection. These levels varied slightly depending on the brand of the booster dose, but we don’t fully understand the meaning of the differences.

Your original vaccine regimen still provides excellent protection against severe disease and death. We will continue to analyze information on the effect of COVID-19 booster shots throughout your participation in the study.

The early results of the trial have been posted online and submitted to a medical journal for publication. The draft paper can be found at the link below:


We will update you with new information as it becomes available. If you are part of a study group that was enrolled after those described above, please rest assured that we will review data collected from you in the near future.

Once again, your participation in this clinical trial is critical, and we thank you for your willingness to be a COVID-19 booster vaccine study volunteer.

The NIAID Mix and Match Study Team