Mix and match of the second COVID-19 vaccine dose for SAfety and ImmunogeniCity

As COVID-19 vaccine programs continue to roll out, global supply and manufacturing interruptions have led to temporary vaccine shortages. This has led some provinces to change, or consider changing, their vaccine rollout strategy to ensure vaccine programs can continue to operate.

One option when there is a shortage of vaccines is to use two different COVID-19 vaccines for the first and second dose. In this case, if a different vaccine is available it could be used rather than waiting for the same kind of vaccine. Combinations of different vaccines were not used in original clinical trial data and is therefore up to researchers and Public Health to help answer questions that will help address vaccine supply shortages.

A second strategy is to extend the wait between the first and second dose (interval) of an approved COVID-19 vaccine from 4 weeks, as recommended by the manufacturer, to up to 16 weeks. Based on clinical trial evidence that shows high efficacy of a single dose and the benefits of having as many people with at least one dose as possible, this strategy is already in use across Canada.  

The objective of the Canadian MOSAIC study is to systematically evaluate these strategies as well to find out how long vaccine protection lasts when these strategies are used. 

Currently, four COVID-19 vaccines made by different manufacturers have received regulatory approval in Canada (Pfizer/BioNTech, Moderna, AstraZeneca, and Johnson and Johnson); three are being administered in public health programs (Pfizer/BioNTech, Moderna, AstraZeneca). As additional vaccines become available and begin being distributed, they will be added to the study. The study may also be modified by adding new populations, such as younger age groups or pregnant individuals, over time. The study data will be communicated regularly to public health and the COVID-19 Immunity Task Force to inform decision making during the pandemic.

The study is led by the Clinical Trials Network of the Canadian Immunization Network (CIRN) and funded by the COVID-19 Immunity Task Force. It is led by Dr. Joanne Langley and Dr. Manish Sadarangani.

Primary Investigators

Dr. Joanne Langley- National Investigator 
Dr. Joanne Langley is a Professor of Pediatrics and Community Health and Epidemiology at Dalhousie University and the Canadian Center for Vaccinology in Halifax, NS Canada, head of Pediatric Infectious Diseases at the IWK Health Centre, and lead for the Clinical Trials Network of the Canadian Immunization Research Network. She currently co-chairs the Canadian COVID-19 Vaccine Task Force. Her research is focused on the epidemiology and vaccine prevention of respiratory infections, particularly Respiratory Syncytial Virus and influenza, and immunization decision-making.

Dr. Guillaume Poliquin, Acting Vice President, National Microbiology Laboratory- Local Investigator 
Dr. Poliquin completed his medical degree at Western University prior to pursuing a pediatrics residency at the University of Manitoba, followed by a fellowship in pediatric infectious diseases also at the University of Manitoba. After residency, Dr. Poliquin joined the Public Health Agency of Canada’s National Microbiology Laboratory’s (NML) Special Pathogens Division for a Ph.D. focused on Ebola virus. He has since assumed the role of Medical Advisor to the Scientific Director General at the NML, and is now acting Vice President. These roles are rounded out with a pediatric infectious diseases consultative practice in Winnipeg, as well as general pediatrics practice in remote communities in northern Manitoba and Nunavut. Dr. Poliquin’s research portfolio is primarily focused on vaccine research and emerging pathogens, such as Ebola and, more recently, SARS-CoV-2.

Dr Guillaume Poliquin, vice-président par intérim du Laboratoire national de microbiologie
Le Dr Poliquin détient un diplôme en médecine de l’Université Western. Il a effectué sa résidence en pédiatrie à l’Université du Manitoba et a ensuite obtenu une bourse de recherche en maladies infectieuses pédiatriques, également de l’Université du Manitoba. Après sa résidence, le Dr Poliquin s’est joint à la Division des agents pathogènes spéciaux du Laboratoire national de microbiologie (LNM) de l’Agence de la santé publique du Canada pour rédiger sa thèse de doctorat sur la maladie à virus Ebola. Il a assumé le rôle de conseiller médical auprès du directeur général scientifique du LNM, et occupe désormais le poste de vice-président par intérim. Outre ces fonctions, le DPoliquin est consultant en maladies infectieuses pédiatriques à Winnipeg et pratique la pédiatrie générale dans les communautés éloignées dans le nord du Manitoba et au Nunavut. Le portefeuille de recherche du DPoliquin porte principalement sur les vaccins et les agents pathogènes émergents, comme le virus Ebola et, plus récemment, le SARS-CoV-2.

Dr. Manish Sadarangani- National Investigator
Dr. Manish Sadarangani is Director of the Vaccine Evaluation Center at the BC Children’s Hospital Research Institute and an Associate Professor in the Division of Infectious Diseases, UBC Department of Pediatrics.

About the Study

Study Purpose and Objectives

MOSAIC will be looking to find the answers to three questions:

  1. How safe and effective is a full COVID-19 vaccination if the first and second dose is 28 days apart compared to a schedule with the first and second doses are 112 days apart.

  2. How safe and effective is a two dose COVID-19 vaccine schedule when different vaccines are used for the first dose and second dose

  3. How long does protection from COVID-19 vaccines last in the above conditions

Study Design

Approximately 1300 participants will be enrolled in the MOSAIC study. Most participants, approximately 80 percent, will be in the “vaccine exposed” group, meaning they have already received their first dose of an approved COVID-19 vaccine. The remaining 20 percent of participants will be “vaccine naive” meaning they have not received any COVID-19 vaccinations. These groups will be put into smaller sub-groups based on age and randomly assigned to one of 13 study groups.

MOSAIC will follow all provincial Public Health recommendations regarding vaccinations. AstraZeneca will not be given as a first dose as part of this study. Those who have already received AstraZeneca as their first dose from a public program will be assigned to the vaccine-exposed group for a second randomized dose.

Interval: 28 Days Apart
Manufacturer’s Recommendation

Interval: 112 Days Apart
Extended Interval

Group 1

1st Dose: Moderna
2nd Dose: Moderna

Group 2

1st Dose: Moderna
2nd Dose: Moderna

Group 3

1st Dose: Moderna 
2nd Dose: Pfizer/BioNTech

Group 4

1st Dose: Moderna
2nd Dose: Pfizer/BioNTech 

Group 5

1st Dose: Pfizer/BioNTech
2nd Dose: Pfizer/BioNTech

Group 6

1st Dose: Pfizer/BioNTech
2nd Dose: Pfizer/BioNTech

Group 7

1st Dose: Pfizer/BioNTech
2nd Dose: Moderna

Group 8

1st Dose: Pfizer/BioNTech
2nd Dose: Moderna 


Group 9

1st Dose: Astra Zeneca
2nd Dose: Astra Zeneca

Group 10

1st Dose: Astra Zeneca
2nd Dose: Moderna

Group 11

1st Dose: Astra Zeneca
2nd Dose: Moderna

Group 12

1st Dose: Astra Zeneca
2nd Dose:  Pfizer/BioNTech

Group 13

1st Dose: Astra Zeneca
2nd Dose:  Pfizer/BioNTech


You can take part in this study if:

  1. You are 18 years of age or older.
  2. You are available for all the study visits.
  3. You are in good health, including if you have stable health conditions.
  4. You are not pregnant.
  5. You have a record of the first dose of the authorized COVID-19 vaccine you received and have not received the second dose yet, OR, you have not received a COVID-19 vaccine.

What will you be asked to do?

You will have 5 clinic visits and 2 telephone/email contacts if you are a vaccine naïve participant (have not received a COVID-19 vaccine) during a 12-month period. The total time commitment is approximately 3-4 hours.

You will have 4 clinic visits and 1 telephone/email contact if you are a vaccine-exposed participant (received your first COVID-19 vaccine from Public Health) during a 11-month period. The total time commitment is approximately 2-3 hours.

You will also be asked to record a diary, where you will note information regarding your symptoms for the first 7 days after your vaccine, and to complete a paper questionnaire twice over the study regarding your opinion about the vaccines.

The following procedures will be done at the study visits and contacts:

  • Consent
  • Medical History
  • Vital Signs
  • Demographic Information
  • Blood Sample
  • Pregnancy Test if indicated
  • Vaccinations

How to Participate

Please contact our study site coordinator directly to learn how you can get involved. 

Email: or phone: 204-789-3206


Partners and Sponsors

Canadian Immunization Research Network

The Canadian Immunization Research Network (CIRN) is a national network of key vaccine researchers who develop, and test methodologies related to the evaluation of vaccines as they pertain to safety, immunogenicity and effectiveness, and program implementation and evaluation. CIRN’s objective is to further strengthen Canada’s research capacity, evidence base and expertise in the field of immunization and vaccines for vaccine-preventable diseases. A ‘network or networks’, CIRN plays a pivotal role in mentoring early-career researchers, recruiting new investigators, providing opportunities for trainees, and delivering meaningful engagement of stakeholders at all research stages.
CIRN is made up of the following 8 networks: the Canadian National Vaccine Safety Network (CANVAS); the Clinical Trials Network (CTN); the Serious Outcomes Surveillance Network (SOS); the Special Immunization Clinic Network (SIC); the Provincial Collaborative Network (PCN); the Social Sciences and Humanities Network (SSHN); the Modeling and Economics Research Network (ModERN); and the Reference Laboratory Network (RLN).

Dalhousie University

Dalhousie University is Atlantic Canada’s leading research-intensive university. Located in the heart of Halifax, Nova Scotia, with an Agricultural Campus in Truro/Bible Hill, Dalhousie is a truly national and international university, with more than half of the university’s 20,000-plus students coming from outside the province. Dal’s 6,000 faculty and staff foster a diverse, purpose-driven community, one that spans 13 faculties and conducts more than $181 million in research annually. Part of a cluster identified as one of the world’s top international centers in ocean research, the university proudly celebrated its 200th anniversary in 2018.

Vaccine Surveillance Reference Group

The Vaccine Surveillance Reference Group (VSRG) supports the monitoring of the safety and effectiveness of COVID-19 vaccines in Canada. It is a consortium of Canadian organizations – the Public Health Agency of Canada (PHAC), the Canadian Research Immunization Network (CIRN), the National Advisory Committee on Immunization (NACI) and the COVID-19 Immunity Task Force (CITF) – working collaboratively to pool expertise on vaccine surveillance. The VSRG reports to the President of the Public Health Agency of Canada (PHAC) and is supported by the COVID-19 Immunity Task Force (CITF) Secretariat. It is co-chaired by the leaders of the National Advisory Committee on Immunization (NACI) and the Canadian Immunization Research Network (CIRN). Among its responsibilities, the VSRG, through the CITF Executive Committee, makes recommendations to PHAC on funding research teams that can address important aspects of the immune response, safety, and effectiveness of COVID-19 vaccines with public health relevance and with attention to all priority groups.

Learn more 

COVID Immunity Task Force (CITF)

In late April 2020, the Government of Canada established the COVID-19 Immunity Task Force with a two-year mandate. The Task Force is overseen by a Leadership Group of volunteers that includes leading Canadian scientists and experts from universities and healthcare facilities across Canada who are focused on understanding the nature of immunity arising from the novel coronavirus that causes COVID-19. To that end, the CITF is supporting numerous studies to determine the extent of SARS-CoV-2 infection in Canada (in the general population as well as in specific communities and priority populations), understand the nature of immunity following infection, develop improved antibody testing methods, and help monitor the effectiveness and safety of vaccines as they are rolled out across Canada. The Task Force and its Secretariat accordingly work closely with a range of partners, including governments, public health agencies, institutions, health organizations, research teams, other task forces, and engages communities and stakeholders. Most recently, the Task Force has been asked to support vaccine surveillance, effectiveness and safety as part of its overall objective to generate data and ideas that inform interventions aimed at slowing – and ultimately stopping – the spread of SARS-CoV-2 in Canada.

Learn more