This is a summary, written by members of the CITF Secretariat, of:

Quach C and Deeks S. COVID-19 vaccination: why extend the interval between doses? Official Journal of the Association of Medical Microbiology and Infectious Disease. 2021 March 25. doi: 10.3138/jammi-2021-0323

The results and/or conclusions contained in the research do not necessarily reflect the views of all CITF members.

SARS-CoV-2 has caused worldwide disruption, morbidity and mortality. Although several vaccines have been approved by regulatory bodies, many countries have been faced with the issue of vaccine shortages. The current COVID-19 vaccines are complex as several require two doses taken weeks apart. Early studies have shown that one dose can be effective in delivering protection, but ultimately two doses are preferred. Many governments have had to make the difficult decision of delaying the second dose to maximize the number of people protected with the first dose. In the Official Journal of the Association of Medical Microbiology and Infectious Disease, Drs. Caroline Quach and Shelley Deeks, respectively Chair and Vice-Chair of the National Advisory Committee on Immunization (NACI), provide further explanation behind extending the interval between doses of currently authorized COVID-19 vaccines to up to four months.

Dr. Quach is a CITF Leadership Group member and both Drs. Quach and Deeks are also core members of the Vaccine Surveillance Reference Group (VSRG).


NACI is an independent body to the Public Health Agency of Canada (PHAC). It is comprised of a panel of experts in various fields that make recommendations for the use of current and newly approved vaccines in Canada. NACI experts make recommendations based on evidence regarding burden of illness, vaccine characteristics (safety, efficacy, and immunogenicity), principles of immunology, and historical experience with vaccines. They also take into consideration the ethics, equity, feasibility, and acceptability of vaccines. In providing direction for the COVID-19 public health emergency, NACI uses multiple direct and indirect evidence-based methods to inform its recommendations regarding COVID-19 vaccines.

On March 3, 2021, NACI issued a strong recommendation to allow for an extended interval of up to four months between vaccine doses to maximize the number of people protected from SARS-CoV-2 via vaccination as quickly as possible. Delaying the second dose to a time when the vaccine supply chain is expected to be more robust ensures that many more people can get their first dose. This decision was based on a review of available data as well as recommendations from other jurisdictions. There was evidence from Quebec and British Columbia that effectiveness did not decrease within the eight weeks following the first vaccine dose. Public Health England also found that vaccine effectiveness in the United Kingdom was 70-80% with no evidence of decline over the four weeks studied. Furthermore, mathematical modelling developed by PHAC showed that intervals of up to six months reduced symptomatic disease, mortality, and hospitalizations, even under assumptions of waning protection. The “up to four months” interval recommendation takes into consideration that NACI will monitor and change the interval based on emerging evidence.

The authors concluded that there is a need for transparency and ongoing surveillance of Canadian and international experiences as well as timely research to ensure informed decisions are made in an appropriate manner during this pandemic.