COVID-19 Vaccine
4th Dose Study

MOSAIC 2: Mix-and-Match of the first two doses plus 3rd booster dose of approved COVID-19 vaccines
MOSAIC 1: Mix-and-Match of first two doses of approved COVID-19 Vaccines

About MOSAIC 3

Recent evidence has shown the effectiveness of COVID-19 vaccines against severe outcomes up to 6 months after vaccine. However, there is some evidence of decreasing effectiveness after six months. In Canada the National Advisory Committee on Immunization (NACI) recommended a booster dose ( third dose) since the fall of 2021. NACI now recommends a second booster dose (fourth dose), with priority be given to vulnerable populations.

MOSAIC 3 will evaluate the safety, tolerability and immune response to a fourth (4th) dose of a COVID-19 vaccine currently authorized in Canada (either the Pfizer-BioNTech or Moderna mRNA vaccine or the Medicago VLP vaccine) in persons who received their last COVID-19 vaccine at least 3 (three) months ago.

MOSAIC 3 is taking place at multiple clinical sites in Canada. View our study locations page to find the site nearest you!


You can take part in this study if:

  1. You are 18 years of age and older
  2. You are available for all the study visits.
  3. You are in good health with stable health conditions.
  4. You are not pregnant.
  5. You received your 3rd dose of an approved COVID-19 vaccine more than three ( 3 ) months from the first study visit
  6. You have a record of your previous doses of the authorized COVID-19 vaccines you received.

There are other study criteria that will be reviewed at the first visit to determine if you are eligible or not.

If you are under 30 years of age, you will not be able to be randomized to the Moderna vaccine group. 

This is due to the potential increased risk of pericarditis/myocarditis and the public health recommendation to persons in this age group to not receive this vaccine.

What will I be asked to do?

You will have 1 vaccine dose, 4 clinic visits and 1 telephone/email contact during a 12-month period.  The total time commitment is approximately 2-3 hours.

If you are enrolled in MOSAIC-2, you will not be scheduled for any more MOSAIC-2 study visits and instead have MOSAIC-3 study visits only.

If you enroll in the participant-blinded part of the study, you will not be able to choose which vaccine you receive.  It is important to stay blinded until at least 28 days after the vaccine dose. You will only be told which vaccine you received at the end of study. If there is an emergency, or if it is required for documentation due to public health measures or employment, we will record the date you saw which vaccine you received. This way, the results from the different groups will be handled in the same way.

If you enroll in the randomized part of the study, this means that your vaccine group will be chosen at random (like rolling a dice). You have an equal chance of being placed in each group.

If you enroll in the open-label part of the study, this means that you can choose to be in the Medicago vaccine group.

MOSAIC-3 Study Groups

Randomized Groups
Previous COVID-19 Doses
Study Vaccine
Dose 4
1c3 doses of Pfizer/BioNTech mRNAPfizer/BioNTech mRNA
2c3 doses of Pfizer/BioNTech mRNAMedicago VLP
3c3 doses of Moderna mRNAModerna mRNA
4c3 doses of Moderna mRNAMedicago VLP
5c3 doses of any authorized COVID-19 VaccineAny authorized mRNA COVID-19 Vaccine
6c3 doses of any authorized COVID-19 VaccineMedicago VLP
Open-Label Group
Previous COVID-19 Doses
Study Vaccine Dose 4
7c3 doses of any authorized COVID-19 vaccineMedicago VLP

Participants in group 7c who have received 3 doses of any authorized COVID-19 vaccine, with the third dose administered at least 3 months prior, will be enrolled and receive an unblinded single dose of the Medicago VLP vaccine.