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

Dick DW, Childs L, Feng Z, Li J, Röst G, Buckeridge DL, Ogden NH, Heffernan JM. Fall 2021 Resurgence and COVID-19 Seroprevalence in Canada Modelling waning and boosting COVID-19 immunity in Canada A Canadian Immunization Research Network Study. medRxiv 2021.08.17.21262188; doi:

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

A study out in preprint in August (and therefore not yet peer-reviewed) led by CITF Scientific Advisor Dr. Jane Heffernan and which included CITF Scientific Lead, Data Management & Analysis, Dr. David Buckeridge, modelled immunity to COVID-19 across Canada. The scientists used a mathematical model to determine the distribution of immunity in the Canadian population and showed that the current level of immunity may not be sufficient to prevent a new COVID-19 wave. Their immunity modelling predictions go until March 2022. The authors suggested that booster vaccines may be needed and emphasized the continuous need for non-pharmaceutical interventions (such as use of masks and social distancing).

The scientists used a mathematical model to determine the distribution of immunity in the Canadian population, considering age, as well as the duration of immunity acquired from infection and/or vaccination.

Key points:

  • The model was programmed using daily COVID-19 incidence data until the end of June 2021 and made a projection through September 2021. Predictions were made until March 2022.
  • Distributions of immunity from January 2020 to March 2022 were quantified, given different assumed characteristics of the vaccines against various variants of concern (i.e., protection from infection, protection from severe disease), and different rates of immunity decay over time.
  • Their model accurately predicted that 60−80% of the Canadian population would have some immunity to COVID-19 by the end of summer 2021. Actual numbers suggested that almost 70% of the Canadian population were fully vaccinated as of September 1st, as recorded by the Our World in Data vaccine tracker.

The authors report that the magnitude of COVID-19 resurgence between Fall 2021 to Winter 2022 will be sensitive to how fast the acquired immunity decays over time, whether non-pharmaceutical interventions and social distancing measures are relaxed, the transmissibility of the circulating virus, and on vaccine effectiveness against infection and disease. If indeed, immunity diminishes or is lost over time, a vaccine booster campaign or the re-introduction of public health mitigation measures will be required.